Fix: return when there are no matched clusters (skypilot-org#500)

Author: suquark

Diff for: sky/cli.py

@@ -1176,6 +1176,7 @@ def _terminate_or_stop_clusters(names: Tuple[str], apply_to_all: Optional[bool],
             names = []
     if not to_down and not names:
         print('No existing clusters found (see `sky status`).')
+        return
 
     if not no_confirm:
         teardown_verb = 'Terminating' if terminate else 'Stopping'