FIX: Drop JSON sidecar fields with null values during metadata indexing

bids/layout/index.py

@@ -465,6 +465,10 @@ def create_association_pair(src, dst, kind, kind2=None):
 
             # Create Tag <-> Entity mappings, and any newly discovered Entities
             for md_key, md_val in file_md.items():
+                # Treat null entries (deserialized to None) as absent
+                # Alternative is to cast None to null in layout.models._create_tag_dict
+                if md_val is None:
+                    continue
                 tag_string = '{}_{}'.format(bf.path, md_key)
                 # Skip pairs that were already found in the filenames
                 if tag_string in all_tags: