fix: handle empty time entries and add missing mypy override for openpyxl

- Treat empty list from fetch_issues_with_spent_time as 'no entries' and exit 0
  instead of crashing later in the pipeline.
- Update CLI output-extension tests to use non-empty mock data so they actually
  reach the extension validation code path.
- Add openpyxl to mypy ignore_missing_imports overrides.

Closes #36

pyproject.toml

@@ -57,5 +57,5 @@ multi_line_output = 3
 warn_unused_configs = true
 
 [[tool.mypy.overrides]]
-module = ["odf.*", "redminelib.*", "tabulate"]
+module = ["odf.*", "redminelib.*", "tabulate", "openpyxl.*"]
 ignore_missing_imports = true