fix type hint

Author: AdeelH

rastervision_core/rastervision/core/data/label_store/semantic_segmentation_label_store.py

@@ -351,7 +351,7 @@ def _clip_to_extent(self,
                         extent: Box,
                         window: Box,
                         arr: Optional[np.ndarray] = None
-                        ) -> Tuple[Box, np.ndarray]:
+                        ) -> Tuple[Box, Optional[np.ndarray]]:
         clipped_window = window.intersection(extent)
         if arr is not None:
             h, w = clipped_window.size