cleanup DatasetItem methods

Author: Drew Kaul

nucleus/dataset_item.py

@@ -96,10 +96,12 @@ class DatasetItem:  # pylint: disable=R0902
     pointcloud_location: Optional[str] = None
 
     def __post_init__(self):
-        self.local = is_local_path(self.image_location)
         assert bool(self.image_location) != bool(
             self.pointcloud_location
         ), "Must specify exactly one of the image_location, pointcloud_location parameters"
+        self.local = (
+            is_local_path(self.image_location) if self.image_location else None
+        )
         self.type = (
             DatasetItemType.IMAGE
             if self.image_location
@@ -143,26 +145,25 @@ def to_payload(self, is_scene=False) -> dict:
         payload: Dict[str, Any] = {
             METADATA_KEY: self.metadata or {},
         }
+        if self.reference_id:
+            payload[REFERENCE_ID_KEY] = self.reference_id
+        if self.item_id:
+            payload[DATASET_ITEM_ID_KEY] = self.item_id
 
         if is_scene:
             if self.image_location:
                 payload[URL_KEY] = self.image_location
             elif self.pointcloud_location:
                 payload[URL_KEY] = self.pointcloud_location
             payload[TYPE_KEY] = self.type.value
+            if self.camera_params:
+                payload[CAMERA_PARAMS_KEY] = self.camera_params.to_payload()
         else:
             assert (
                 self.image_location
-            ), "Must specify image_location for DatasetItems not in a Scene"
+            ), "Must specify image_location for DatasetItems not in a LidarScene"
             payload[IMAGE_URL_KEY] = self.image_location
 
-        if self.reference_id:
-            payload[REFERENCE_ID_KEY] = self.reference_id
-        if self.item_id:
-            payload[DATASET_ITEM_ID_KEY] = self.item_id
-        if self.camera_params:
-            payload[CAMERA_PARAMS_KEY] = self.camera_params.to_payload()
-
         return payload
 
     def to_json(self) -> str: