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

[github-actions]

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