Made ChannelDetails.status and ChannelStatus.occupancy optional

[maratal]

As mentioned by @lawrence-forooghian in this and this comments, both ChannelDetails.status and ChannelStatus.occupancy should be optional.

[owenpearson]

Hey @maratal, these types should actually stay non-nullable - the confusion here is actually down to the documentation for ChannelStatus being misleading. The type is used for lifecycle events sent with the channel.lifecycle reactor source (see here), so these fields are documented as nullable because in that context they are. When using the REST API these fields will always exist so you should leave the spec as it is and change the ably-cocoa PR to make these fields non-nullable.

I've created #1456 to address this on the docs side.