Return none if a schema path is not found

This can cause overwriting of unintended keys, so should not be
happening. Will have to find the bugs this causes.

daemon-common/zkhandler.py

@@ -700,7 +700,8 @@ class ZKSchema(object):
                 base_path = ''
             sub_path = self.schema.get(itype).get('.'.join(ipath))
             if sub_path is None:
-                sub_path = ''
+                # We didn't find the path we're looking for, so we don't want to do anything
+                return None
             return f'{base_path}/{item}{sub_path}'
 
     # Get keys of a schema location