fix: Use correct import for @Nullable annotation.

[bdferris-v2]

I accidentally included the wrong @nullable annotation on a new class (used org.jetbrains.annotations.Nullable when I should have used javax.annotation.Nullable). We should be consistent with the rest of the codebase.

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[davidgamez]

question: why do we need to import this library?

[bdferris-v2]

The @nullable annotation is not included in the JDK by default so it has to be included via a dependency. com.google.code.findbugs:jsr305 is actually where we've been getting this annotation in core and main, but it was a transitive dependency via com.google.guava:guava. Since model doesn't need Guava, I just imported the dependency directly.

[github-actions]

✅ Rule acceptance tests passed.
New Errors: 0 out of 1427 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Errors: 0 out of 1427 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
New Warnings: 0 out of 1427 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
Dropped Warnings: 0 out of 1427 datasets (~0%) are invalid due to code change, which is less than the provided threshold of 1%.
0 out of 1427 sources (~0 %) are corrupted.
Commit: 91d3316
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✅ Rule acceptance tests passed.