fix voiceChannelLeave event on no cache

[urbainn]

hello,

when caching is disabled, the voiceChannelLeave event does not emit at all. that's because the event is only emitted if the cached value oldChannel is defined. instead, I propose to directly check if the state.channelID is null, so the event doesn't have to depend on whether the old voice state and the old channel are cached.

when the old channel is not cached, it will pass null as the value for the oldChannel event parameter, so I updated its type accordingly

thanks