Fix humidity warning

[GrimmiMeloni]

HM-IP-eTRV2 is detected as ThermostatServiceIP, however it does not have a humidity value.
This causes some unnecessary warnings in the logs, which this change fixes.

[bcschwede]

Hi there,
I see warnings of this kind in my homebridge logs: " [homebridge-homematic] This plugin generated a warning from the characteristic 'Current Relative Humidity': characteristic value expected valid finite number and received "NaN" (number). See https://homebridge.io/w/JtMGR for more info."
Probably you solved this problem - can i be of any help in testing? How can I install this fix?
Best,
BC

[GrimmiMeloni]

Correct, this fix takes care of this warning.
Not sure if testing is required. It would take @thkl to accept the PR into his repo and create a new release of the plugin.
Alternatively you can checkout this branch and install the plugin manually into home bridge.

[redtom13]

Tested and worked