Fix: coalesce the max download count in case the database is empty

[netomi]

When setting up databasesearch in a dev environment with an empty database, you get an exception like that:

Caused by: org.springframework.aop.AopInvocationException: Null return value from advice does not match primitive return type for: public abstract int org.eclipse.openvsx.repositories.ExtensionRepository.getMaxDownloadCount()
	at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:237) ~[spring-aop-6.2.11.jar:6.2.11]
	at jdk.proxy4/jdk.proxy4.$Proxy226.getMaxDownloadCount(Unknown Source) ~[na:na]
	at org.eclipse.openvsx.repositories.RepositoryService.getMaxExtensionDownloadCount(RepositoryService.java:163) ~[main/:na]
	at org.eclipse.openvsx.search.RelevanceService$SearchStats.<init>(RelevanceService.java:150) ~[main/:na]
	at org.eclipse.openvsx.search.DatabaseSearchService.sortExtensions(DatabaseSearchService.java:81) ~[main/:na]
	at org.eclipse.openvsx.search.DatabaseSearchService.search(DatabaseSearchService.java:66) ~[main/:na]
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[na:na]
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
	at java.base/java.lang.reflect.Method.invoke(Method.java:568) ~[na:na]
	at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:360) ~[spring-aop-6.2.11.jar:6.2.11]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:196) ~[spring-aop-6.2.11.jar:6.2.11]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163) ~[spring-aop-6.2.11.jar:6.2.11]
	at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:380) ~[spring-tx-6.2.11.jar:6.2.11]
	at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:119) ~[spring-tx-6.2.11.jar:6.2.11]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184) ~[spring-aop-6.2.11.jar:6.2.11]
	at org.springframework.cache.interceptor.CacheInterceptor.lambda$invoke$0(CacheInterceptor.java:55) ~[spring-context-6.2.11.jar:6.2.11]
	at org.springframework.cache.interceptor.CacheAspectSupport.invokeOperation(CacheAspectSupport.java:431) ~[spring-context-6.2.11.jar:6.2.11]
	at org.springframework.cache.interceptor.CacheAspectSupport.evaluate(CacheAspectSupport.java:601) ~[spring-context-6.2.11.jar:6.2.11]
	at org.springframework.cache.interceptor.CacheAspectSupport.execute(CacheAspectSupport.java:448) ~[spring-context-6.2.11.jar:6.2.11]
	at org.springframework.cache.interceptor.CacheAspectSupport.execute(CacheAspectSupport.java:410) ~[spring-context-6.2.11.jar:6.2.11]
	at org.springframework.cache.interceptor.CacheInterceptor.invoke(CacheInterceptor.java:65) ~[spring-context-6.2.11.jar:6.2.11]
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:184) ~[spring-aop-6.2.11.jar:6.2.11]
	at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:728) ~[spring-aop-6.2.11.jar:6.2.11]
	at org.eclipse.openvsx.search.DatabaseSearchService$$SpringCGLIB$$0.search(<generated>) ~[main/:na]
	at org.eclipse.openvsx.search.SearchUtilService.search(SearchUtilService.java:62) ~[main/:na]
	at org.eclipse.openvsx.LocalRegistryService.search(LocalRegistryService.java:260) ~[main/:na]
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[na:na]
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
	at java.base/java.lang.reflect.Method.invoke(Method.java:568) ~[na:na]
	at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:360) ~[spring-aop-6.2.11.jar:6.2.11]
	at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:724) ~[spring-aop-6.2.11.jar:6.2.11]
	at org.eclipse.openvsx.LocalRegistryService$$SpringCGLIB$$0.search(<generated>) ~[main/:na]
	at org.eclipse.openvsx.RegistryAPI.search(RegistryAPI.java:786) ~[main/:na]
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77) ~[na:na]
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
	at java.base/java.lang.reflect.Method.invoke(Method.java:568) ~[na:na]
	at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:258) ~[spring-web-6.2.11.jar:6.2.11]
	at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:191) ~[spring-web-6.2.11.jar:6.2.11]
	at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:118) ~[spring-webmvc-6.2.11.jar:6.2.11]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:991) ~[spring-webmvc-6.2.11.jar:6.2.11]
	at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:896) ~[spring-webmvc-6.2.11.jar:6.2.11]
	at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-6.2.11.jar:6.2.11]
	at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1089) ~[spring-webmvc-6.2.11.jar:6.2.11]
	at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:979) ~[spring-webmvc-6.2.11.jar:6.2.11]
	at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1014) ~[spring-webmvc-6.2.11.jar:6.2.11]

This PR fixes that by coalescing the max function to 0 if there is no data.

Could not write a unit test as all data is mocked so far.