Added strict null checks for findModel.test.ts

[Pranshu258]

#65233

In this PR:

• Fixed errors: "Object is possibly 'null'.", for tests in file "./vs/editor/contrib/find/test/findModel.test.ts"

[Pranshu258]

There are other errors on running the yarn strict-null-check command, all of them have the following message: Argument of type 'ICodeEditor' is not assignable to parameter of type 'IActiveCodeEditor'.

I am not sure how to fix that, therefore I have not added the file name to src/tsconfig.strictNullChecks.json. It would be great if someone could point me in the right direction to fix that. I will raise another PR for the fix.