fix(spans): Convert nones to zeros for failure rate and count

[Zylphrex]

Nones cause the frontend to render (no value). So convert these to 0s.

[github-actions]

Backend Test Failures

Failures on df57730 in this run:

tests/sentry/discover/test_compare_tables.py::CompareTablesTestCase::test_compare_empty_field_tables — log

tests/sentry/discover/test_compare_tables.py:292: in test_compare_empty_field_tables
    assert (
E   AssertionError: assert (['http.status_code'] is not None and 'failure_rate()' in ['http.status_code'])

[github-actions]

Backend Test Failures

Failures on df5fad5 in this run:

tests/sentry/discover/test_compare_tables.py::CompareTablesTestCase::test_compare_empty_field_tables — log

tests/sentry/discover/test_compare_tables.py:292: in test_compare_empty_field_tables
    assert (
E   AssertionError: assert (['http.status_code'] is not None and 'failure_rate()' in ['http.status_code'])

[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]

Orphaned test fixture left after removing its usage

Low Severity

self.empty_field_widget is still created in setUp but is now unused. The associated empty_field_widget_query and the test_compare_empty_field_tables test that referenced it were both removed in this PR, but the widget fixture itself was left behind. This creates an unnecessary database object on every test run and may confuse future readers.