Magnet value fix

[Johnsel]

There is an issue with the lumi.magnet initial value fetching in combination with the homebridge-miio-gateway and miio library. It returns the first value properly, but then triggers a second event where the value == null. This causes the the state to change to Open, when the actual state is Closed.

Fix: filter out null and undefined values.

[mortenf]

This works.