Actuator Info class has inconsistent nullability annotations and cannot be built with null value

[ngocnhan-tran1996]

From gh-48401, I wrote a test

    @Test
    fun contextLoads() {
        val map = mutableMapOf<String, Any?>("info" to null)
        assertDoesNotThrow { Info.Builder().withDetails(map).build() }
    }

and it failed

> Task :compileTestKotlin FAILED
Argument type mismatch: actual type is 'HashMap<String, Any?>', but '(Mutable)Map<String, out Any>' was expected.
Java type mismatch: expected '(Mutable)Map<String, out Any>' but found 'HashMap<String, Any?>'. Use explicit cast.

This PR fixes the above issue.

[philwebb]

Thanks!