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

redmine_reporter/cli.py

@@ -114,6 +114,10 @@ def main(argv: Optional[List[str]] = None) -> int:
         print("ℹ️  No time entries found in the given period.", file=sys.stderr)
         return 0
 
+    if not issue_hours:
+        print("ℹ️  No time entries found in the given period.", file=sys.stderr)
+        return 0
+
     print(f"✅ Total issues: {len(issue_hours)} [{args.date}]", file=sys.stderr)
 
     rows = build_grouped_report(issue_hours, fill_time=not args.no_time)