fix warn log message (#5422)

Author: lcouzens

koku/masu/util/aws/common.py

@@ -216,8 +216,8 @@ def __init__(self, credentials):
         if match:
             for key, val in match.groupdict().items():
                 setattr(self, key, val)
-
-        LOG.warning(f"Invalid ARN: {self.arn}")
+        else:
+            LOG.warning(f"Invalid ARN: {self.arn}")
 
     def __repr__(self):
         """Return the ARN itself."""