Make sure save_dir can be empty str (#15638)

(cherry picked from commit c53dc20)

Author: tshu-w

src/pytorch_lightning/callbacks/model_checkpoint.py

@@ -574,7 +574,10 @@ def __resolve_ckpt_dir(self, trainer: "pl.Trainer") -> _PATH:
             return self.dirpath
 
         if len(trainer.loggers) > 0:
-            save_dir = trainer.loggers[0].save_dir or trainer.default_root_dir
+            if trainer.loggers[0].save_dir is not None:
+                save_dir = trainer.loggers[0].save_dir
+            else:
+                save_dir = trainer.default_root_dir
             name = trainer.loggers[0].name
             version = trainer.loggers[0].version
             version = version if isinstance(version, str) else f"version_{version}"