text: Title: Cannot save MQTT input configuration changes
  Body: I am using Mycodo 8.9.0. Whenever I add an MQTT input, click the (+) sign and then click "Save" - no changes made at all - I receive the following error :
  
  Error: Modify Input: '<' not supported between instances of 'NoneType' and 'float'
  
  The error happens regardless of which fields I change or attempt to modify (even when no fields are modified as I mentioned above). I'm new to mycodo so please do let me know if I'm doing something wrong. Thanks in advance for your help!