Simplify JavaList.pop and Ensure IndexError on Empty Lists

Summary
This patch refactors the JavaList.pop implementation to default the key parameter to -1 instead of None, removes the special-case branch, and delegates all index handling to __compute_index. As a result, calling pop() on an empty Java list now correctly raises an IndexError in accordance with the MutableSequence contract.

Related Issues

• Lesser fix for ArrayIndexOutOfBoundsException: -1 when running ArrayList.clear() #417

Changes

- Removes the `key is None` branch entirely.
- Relies on `__compute_index` to translate both positive and negative indices (and to raise `IndexError` if out of bounds).

• Test coverage:
  Added comprehensive unit tests in java_list_test.py:

  • testPopDefault: Verifies that pop() removes and returns the last element.

  • testPopPositiveIndex: Covers popping at explicit positive indices (including 0).

  • testPopNegativeIndex: Covers various negative indices (-1, -2, etc.).

  • testPopEdgeCases: Confirms that popping from an empty list or with out-of-bounds indices raises IndexError.

Testing
All existing tests pass, and the new testPop* methods validate correct behavior across default, positive, negative, and edge-case scenarios.

Future Considerations

• We could remove JavaList.pop entirely and inherit MutableSequence.pop, which appears to handle all cases correctly. That approach may reduce maintenance overhead, but risks altering backward-compatibility with any custom behavior in java_remove.