fix subthing id

ml/thing.py

@@ -610,8 +610,6 @@ def _getValue(self, source, uri_list):
 
         # ??? What if the uri points to a Value object?
         # Shouldn't it be serialized?!
-        print(source)
-        print(uri_list)
         value = source[uri_list[0]]
         if uri_list.__len__() == 1:
             # if is ditto-feature
@@ -969,7 +967,8 @@ def to_subthing_json(self):
         if self.thing_id:
             json_out["identifier"] = self.thing_id
         else:
-            json_out["identifier"] = self.model["attributes"]["identifier"]
+            if self.model["attributes"].get("identifier") is not None:
+                json_out["identifier"] = self.model["attributes"]["identifier"]
         for key in self.roles.keys():
             json_out["roles"].append(self.roles[key].to_json())
         for key in self.features.keys():