fixed mypy error

Author: HarryAnkers

src/lightning/pytorch/loggers/mlflow.py

@@ -331,7 +331,11 @@ def after_save_checkpoint(self, checkpoint_callback: ModelCheckpoint) -> None:
         # log checkpoints as artifacts
         if self._log_model == "all" or self._log_model is True and checkpoint_callback.save_top_k == -1:
             self._scan_and_log_checkpoints(checkpoint_callback)
-        elif self._log_model is True and checkpoint_callback not in self._checkpoint_callbacks:
+        elif (
+            self._log_model is True
+            and self._checkpoint_callbacks
+            and checkpoint_callback not in self._checkpoint_callbacks
+        ):
             self._checkpoint_callbacks.append(checkpoint_callback)
 
     def _scan_and_log_checkpoints(self, checkpoint_callback: ModelCheckpoint) -> None: