Fix coverage extension crash when module contents are not found

[Lonami]

Subject: Fix #11678.

Feature or Bugfix

• Bugfix

Purpose

• Fix ZeroDivisionError in _write_py_statistics.
• Running with python -m sphinx -M coverage.

Detail

Fixes:

Exception occurred:
  File "C:\Users\L\Documents\Projects\sphinx\sphinx\ext\coverage.py", line 295, in _write_py_statistics
    f'{100 * len(all_documented_objects) / len(all_objects):.2f}%',
       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
ZeroDivisionError: division by zero

Relates

• 31829a9

[AA-Turner]

Please update CHANGES.

[Lonami]

Not sure if the "Patch by Lonami" is correct. The patch isn't mine. I just applied it and sent it as a standalone PR.

Regardless I plan to make a subsequent PR with more improvements to the coverage module (namely, coverage for parameter documentation). Which will make this one redundant. But I guess it can be merged, in case my later PR is not.

LMK if you want me to squash the changes.