Add nullability annotations to IndexSearcher APIs

[Edarke]

Annotate public APIs with an internal definition of @Nullable where appropriate.

This proposal is a follow up from the discussion here. This change adds an internal copy of JSpecify's @Nullable to lucene-util and uses Intellij's "infer nullability" feature to automatically apply the annotation to IndexSearcher and its dependencies. If this looks good, there will be a follow up PR to automatically annotate other APIs.

The motivation for this change is to increase the null safety of downstream projects using Lucene by allowing users to configure a javac plugin to lint their build. This change was tested with NullAway, which recognizes any annotation with the simple class name @Nullable

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[msokolov]

This seems reasonable to me, but it implies a future promise to maintain it, and I don't know how we would ever know if we added some new usage that isn't properly annotated

[rmuir]

For the java language:

Specifying that something is nullable doesn't provide any value: all types are nullable by default already.

Specifying that something is NOT-nullable would allow for additional smarts: it is the only possible way to do "gradual nullness typing".

I agree too, that we would need linter to verify this stuff in builds. Maybe the errorprone could do it.

[msokolov]

Specifying that something is nullable doesn't provide any value: all types are nullable by default already.

I guess these null-checking systems impose their own assumptions and assume that anything not annotated with @nullable is not-nullable. Clearly the language doesn't enforce that, but maybe some build-time processor can check for it? Also, the proposal here seems to have been to annotate everything all at once, not gradual, but I agree it seems more sensible to assume everything can be null, indicate @NotNull and then enforce that.