fix: #26 check existence of datapoint.poller.func

[kadaliao]

ElementType always creates a dummy poller for those Datapoints without function. (

pytmdb3/tmdb3/util.py

Line 354 in 07c8495

poller = Poller(pollers[k], lookup)

)

So self.poller is not None in Data.__get__ will never be True. (

pytmdb3/tmdb3/util.py

Line 150 in 07c8495

if self.poller is None:

)

It causes problem like #26

Check self.poller.func is not None instead will fix it.

[kadaliao]

And should return the default value for Datapoint if there is no data and no poller function.