refactor: remove dead code in cli and xlsx

cli.py: drop redundant 'if issue_hours is None' branch before
'if not issue_hours'. The branch is not formally dead (client
returns None when no time entries found, client.py), but it is a
semantic duplicate: both branches print the same message and
return 0, and None is falsy, so a single 'not issue_hours' check
covers None, empty list and keeps behavior identical.
test_cli_returns_zero_on_no_entries stays green unchanged.

xlsx.py: drop unreachable 'if ws is None' fallback. Workbook()
(write_only=False) always creates one worksheet in __init__, so
wb.active is never None; set the title directly.

Closes #63

redmine_reporter/cli.py

@@ -422,10 +422,6 @@ def main(argv: Optional[List[str]] = None) -> int:
             logging.exception("Unexpected error")
         return 1
 
-    if issue_hours is None:
-        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