Improve voice state event selection logic

[tombuente]

First, we verify if the old state is valid and whether its channel ID is not nil. This indicates that the user has left a voice channel. Next, we check if the event includes a new channel ID. This indicates that the user has joined a new voice channel.
oldOk && oldVoiceState.ChannelID != nil && event.ChannelID != nil

The first part ((oldOk || oldVoiceState.ChannelID == nil)) of the next comparison is redundant. We already confirmed whether a move took place, so if the event's channel ID is not nil, it must be a join.

In my opinion the last comparison is flawed. We don't really check whether there was an old voice state. If the old state is invalid, we have not left a channel. According to the old logic, a leave still takes place, even if only the new channel ID is not equal to nil.