Let API deal with empty json quantities.

rest-api-service/app/dmss.py

@@ -22,7 +22,10 @@ _default_per_page = 25
 
 # transforms
 def json_str(value):
-    return json.loads(value)
+    if value is None or value == '':
+        return None
+    else:
+        return json.loads(value)
 
 
 def int_id(value):