Homee: fix cover if it has no up/down attribute

[Taraman17]

Breaking change

Proposed change

This fixes Homee cover platform for cover nodes that have no up/down or open/close control but only a position slider or an impulse switch.

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[joostlek]

you can do assert self._open_close_attribute is not None

[Taraman17]

What will happen with the AssertionError then in case no attribute is there?
Do I have to do try/except with it?

[Taraman17]

Also the python reference states, that it is intended for debugging.