fix null checks by appending '--'

app/pdb.py

@@ -1177,7 +1177,7 @@ def fetch_high_level_interactions(pdb_id, bm_id, entity_id=None):
     query = """
     MATCH (e:Entry {ID:$pdb_id})-[:HAS_BOUND_MOLECULE]->(bm:BoundMolecule {ID:$bm_id})<-[r:IS_PART_OF]-(bl1:BoundLigand)<-[:IS_AN_INSTANCE_OF]-(entity1:Entity###)
     OPTIONAL MATCH (bl1)-[contact:HAS_ARP_CONTACT]-(x)
-    WHERE r.OPERATOR = contact.SYM_OP_1
+    WHERE COALESCE(r.OPERATOR,'--') = COALESCE(contact.SYM_OP_1,'--')
     RETURN bm.AU_FALLBACK, entity1.ID, bl1.CHEM_COMP_ID, contact.AUTH_ASYM_ID_1, bl1.PDB_INS_CODE, contact.AUTH_SEQ_ID_1, contact.SYM_OP_1, contact.ATOM_1, x.CHEM_COMP_ID,
     contact.AUTH_SEQ_ID_2, contact.AUTH_ASYM_ID_2, x.PDB_INS_CODE, contact.SYM_OP_2, contact.ATOM_2,
     contact.CONTACT_TYPE, contact.INTERACTION_TYPE,