Unable to use `logging.NOTSET` with `assertLogs` or `assertNoLogs`

Bug report

Bug description:

Since logging.NOTSET equals 0 (a falsey value), the naive conditional check in _AssertLogsContext prevents it from being used as a valid level parameter with unittest.TestCase.assertLogs() or unittest.TestCase.assertNoLogs().

Example:

import logging
import unittest


class LoggerTestCase(unittest.TestCase):
    def test_logging(self):
        root_logger = logging.getLogger()
        root_logger.setLevel(logging.ERROR)

        logger = logging.getLogger("foo")

        with self.assertLogs(logger=logger, level=logging.NOTSET) as cm:
            logger.info("first message")
            logger.error("second message")
            # This test currently fails because `assertLogs(...)` sees the falsey
            # `logging.NOTSET` and sets the level to `logging.INFO` by default; so we
            # end up seeing the "INFO:foo:first message" in the output.
            self.assertEqual(cm.output, ["ERROR:foo:second message"])

In my real life use case, the logging.NOTSET interaction is useful in context of creating a custom class ExplicitlyConfiguredLogger(logging.Logger) that only logs when the logger is configured with something other than logging.NOTSET (used in https://github.com/element-hq/synapse/pull/18474) and wanting to write some tests for it.

CPython versions tested on:

3.13

Operating systems tested on:

Linux

Linked PRs

• gh-137555

Start in Lib/unittest/_log.py at the _AssertLogsContext conditional referenced in the issue. Reproduce the example with logging.NOTSET through assertLogs and assertNoLogs, then verify that both contexts accept the level without defaulting to INFO and that regression coverage exercises this behavior.