fix: narrow LogRecord.exc_info by truthiness for ty 0.0.89

[lesnik512]

The scheduled dep-check lint job failed on ty 0.0.89 (run 38892705113):

error[not-subscriptable]: Cannot subscript object of type `Literal[False]` with no `__getitem__` method
    --> tests/test_kafka_committer.py:1516:23

ty 0.0.89 ships a newer typeshed where LogRecord.exc_info also includes Literal[False]. On 0.0.88 it was tuple[...] | tuple[None, None, None] | None. The is not None check no longer rules out False, so the subscript on the next line fails.

assert exc_info removes both None and False. At runtime the value is a 3-tuple and always truthy, so the test checks the same thing as before.

Checked locally with ty 0.0.89: just lint-ci passes, and the unit tests pass (218). I didn't run the integration tests because there's no Kafka broker on this machine; CI covers them.