Java: Add more nullness tests and fix a bug causing false negatives.

[aschackmull]

This adds a number of additional tricky control-flow tests, which are useful in my work on control-flow reachability.

I've also uncovered a subtle bug in nullness, which I've fixed and documented with a test.

Finally, a drive-by improvement to Java Guards teaching it about the non-nullness of catch-clause declared exception variables (this is already known by the current nullness library, but there it relies on full SSA, which is why this case didn't originally make it into the Guards instantiation).

[Copilot]

Pull Request Overview

This PR adds enhanced test coverage for nullness analysis control flow scenarios and fixes a subtle bug causing false negatives in nullness detection. The changes include additional complex loop and correlation tests that expose edge cases in control flow analysis, plus a fix to a boolean logic error in the tracking variable guard implementation.

• Adds comprehensive test cases for complex loop scenarios and variable tracking edge cases
• Fixes a boolean logic bug in trackingVarGuard that was causing false negatives
• Enhances Guards library to recognize non-null catch clause exception variables

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
B.java	Adds new test methods covering complex loop scenarios, variable correlation, and edge cases for nullness analysis
NullMaybe.expected	Updates expected test results to include new nullness warnings from the added test cases
Nullness.qll	Fixes boolean logic bug in trackingVarGuard by removing incorrect .booleanNot() call
Guards.qll	Adds logic to recognize catch clause exception variables as non-null

[aschackmull]

Dca looks good - several repos have one or two instances of this, so adding a change note.