Address comments

transmogrifier/sources/transformer.py

@@ -402,22 +402,20 @@ def create_locations_from_spatial_subjects(fields: dict) -> dict:
         Args:
            fields: A dict of fields representing a TIMDEX record.
         """
-        if not (subjects := fields.get("subjects")):
+        # this condition is required when Datacite derives None for subjects
+        if fields.get("subjects") is None:
             return fields
 
         if not (
-            spatial_subjects := list(
-                filter(
-                    lambda subject: subject.kind == "Dublin Core; Spatial", subjects  # type: ignore[arg-type]
-                )
-            )
+            spatial_subjects := [
+                subject
+                for subject in fields.get("subjects", [])  # defaults to empty list
+                if subject.kind == "Dublin Core; Spatial" and subject.value is not None
+            ]
         ):
             return fields
 
         for subject in spatial_subjects:
-            if not subject.value:
-                continue
-
             for place_name in subject.value:
                 subject_location = timdex.Location(value=place_name, kind="Place Name")
                 if subject_location not in fields.setdefault("locations", []):