Fix is_empty when disk tier is not None (#2215)

fix is empty for disk tier

sky/resources.py

@@ -902,6 +902,7 @@ def is_empty(self) -> bool:
             self.accelerator_args is None,
             not self._use_spot_specified,
             self.disk_size == _DEFAULT_DISK_SIZE_GB,
+            self.disk_tier is None,
             self._image_id is None,
         ])