Ensure non-nil sessionId

[SystemKeeper]

• Follow-up to Raise fatalError when we reached an inconsistent state #1789

Fixed in this PR:

• SessionId is an optional, we only rely on the HTTP status code currently
• For creating a room object, we only rely on the listable capability currently
• In ChatViewController we just ensure that there's a NCRoom object, not, that it is the correct one

(The last 2 fixes are fixing basically the same thing, but I think we should be explicit here)

[SystemKeeper]

Most of the observed crashes that were observed with a null-token seem to have gone, which makes we wonder if they were really a bug in our (talk/talk-ios) codebase...
Therefore I think we can remove the fatalError experiment again (didn't yield any results anyway). Still I would like to add the safe-guards, just in case.