Don't report @Nullable type argument errors for unmarked classes

[haewiful]

Thank you for contributing to NullAway!

Please note that once you click "Create Pull Request" you will be asked to sign our Uber Contributor License Agreement via CLA assistant.

Before pressing the "Create Pull Request" button, please provide the following:

• [ ] Changed the GenericsChecks.java and NullAway.java files so that they will consider classes that were annotated by the @NullUnmarked annotation.
• [ ] It is "testUseOfUnannotatedCode()" in the GenericsTests.java file.

[CLAassistant]

All committers have signed the CLA.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 85.94%. Comparing base (9860ab2) to head (06c01c1).

Files	Patch %	Lines
...away/src/main/java/com/uber/nullaway/NullAway.java	80.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #958 +/- ## ============================================ - Coverage 85.94% 85.94% -0.01% - Complexity 2045 2047 +2 ============================================ Files 81 81 Lines 6761 6765 +4 Branches 1302 1305 +3 ============================================ + Hits 5811 5814 +3 Misses 537 537 - Partials 413 414 +1 ```

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[msridhar]

@haewiful CI is failing:

https://github.com/uber/NullAway/actions/runs/9507798037/job/26207945224?pr=958#step:6:506

You can run ./gradlew :nullaway:buildWithNullAway to reproduce. It's failing when we try to run NullAway on itself.