Skip attr reload for 'session'

[jjlawren]

Fix as suggested by @pkkid in #397.

As described there:

When we see None or empty list, we assume the object returned from Plex is incomplete and reload the official full object from the url defined by self.key. The problem here is that new object doesn't store any information about the playing sessions of that object, and they get overwritten with the empty data in Playable().

[pkkid]

Thanks.