ref(cells): Update assert_webhook_payloads_for_mailbox

[lynnagara]

more region to cell renaming

[github-actions]

Backend Test Failures

Failures on 31886c3 in this run:

tests/sentry/middleware/integrations/parsers/test_github.py::GithubRequestParserTest::test_webhook_for_codecov_no_cells — log

[gw1] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/middleware/integrations/parsers/test_github.py:233: in test_webhook_for_codecov_no_cells
    assert_webhook_payloads_for_mailbox(
src/sentry/testutils/outbox.py:83: in assert_webhook_payloads_for_mailbox
    cell_names_set = set(cell_names or region_names)
E   TypeError: 'NoneType' object is not iterable

During handling of the above exception, another exception occurred:
tests/sentry/middleware/integrations/parsers/test_github.py:229: in test_webhook_for_codecov_no_cells
    with pytest.raises(
E   AssertionError: Regex pattern did not match.
E    Regex: 'Missing 1 WebhookPayloads for codecov'
E    Input: "'NoneType' object is not iterable"

tests/sentry/middleware/integrations/parsers/test_github.py::GithubRequestParserTypeRoutingTest::test_webhook_no_codecov_payload_when_forwarding_disabled — log

[gw1] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/middleware/integrations/parsers/test_github.py:274: in test_webhook_no_codecov_payload_when_forwarding_disabled
    assert_webhook_payloads_for_mailbox(
src/sentry/testutils/outbox.py:83: in assert_webhook_payloads_for_mailbox
    cell_names_set = set(cell_names or region_names)
E   TypeError: 'NoneType' object is not iterable

During handling of the above exception, another exception occurred:
tests/sentry/middleware/integrations/parsers/test_github.py:270: in test_webhook_no_codecov_payload_when_forwarding_disabled
    with pytest.raises(
E   AssertionError: Regex pattern did not match.
E    Regex: 'Missing 1 WebhookPayloads for codecov'
E    Input: "'NoneType' object is not iterable"

tests/sentry/middleware/integrations/parsers/test_github.py::GithubRequestParserTypeRoutingTest::test_webhook_for_codecov — log

[gw1] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/middleware/integrations/parsers/test_github.py:193: in test_webhook_for_codecov
    assert_webhook_payloads_for_mailbox(
src/sentry/testutils/outbox.py:83: in assert_webhook_payloads_for_mailbox
    cell_names_set = set(cell_names or region_names)
E   TypeError: 'NoneType' object is not iterable

tests/sentry/middleware/integrations/parsers/test_github.py::GithubRequestParserTest::test_webhook_for_codecov — log

[gw0] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/middleware/integrations/parsers/test_github.py:193: in test_webhook_for_codecov
    assert_webhook_payloads_for_mailbox(
src/sentry/testutils/outbox.py:83: in assert_webhook_payloads_for_mailbox
    cell_names_set = set(cell_names or region_names)
E   TypeError: 'NoneType' object is not iterable

tests/sentry/middleware/integrations/parsers/test_github.py::GithubRequestParserTest::test_webhook_no_codecov_payload_when_forwarding_disabled — log

[gw0] linux -- Python 3.13.1 /home/runner/work/sentry/sentry/.venv/bin/python3
tests/sentry/middleware/integrations/parsers/test_github.py:274: in test_webhook_no_codecov_payload_when_forwarding_disabled
    assert_webhook_payloads_for_mailbox(
src/sentry/testutils/outbox.py:83: in assert_webhook_payloads_for_mailbox
    cell_names_set = set(cell_names or region_names)
E   TypeError: 'NoneType' object is not iterable

During handling of the above exception, another exception occurred:
tests/sentry/middleware/integrations/parsers/test_github.py:270: in test_webhook_no_codecov_payload_when_forwarding_disabled
    with pytest.raises(
E   AssertionError: Regex pattern did not match.
E    Regex: 'Missing 1 WebhookPayloads for codecov'
E    Input: "'NoneType' object is not iterable"

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

^{Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, enable autofix in the Cursor dashboard.}

[cursor]

Falsy-check on cell_names ignores explicit empty list

Low Severity

The expression cell_names or region_names or [] uses Python's truthiness check, which treats an empty list [] the same as None. If a caller explicitly passes cell_names=[] (meaning "expect no cells") while getsentry simultaneously passes region_names=["us"] during the transition period, the empty cell_names would be silently ignored and region_names would be used instead. Using cell_names if cell_names is not None else (region_names if region_names is not None else []) would correctly distinguish "not provided" from "explicitly empty." Current in-repo callers aren't affected because they never pass both parameters, but this could bite getsentry callers during the synchronization window.