_ensureJoined should check for actual membership if cache returns nothing

[austinhuang0131]

In Intent#_ensureJoined, after cache returns anything that's not "join", the intent would immediately try to join the room. However, since cache is cleared on each restart, if cache returns null, then the intent does not know whether it is actually in a room or not. This creates some issues:

• It is not consistent with _ensureHasPowerLevelFor where actual data is checked if cache returns null, and
• Running joinRoom again could override existing membership events, especially custom display name and avatar in the room.

I can provide a pull request. Please reconsider. Thanks!