update session.fromMap() to not throw when passed a session with "sessionId": null

[eth0fox]

Fixes #97

[Nutcake]

This would fix the linked issue but has the potential to cause problems in other places where an empty session id is not expected (Example: Tapping on the session invite will open the session view, which uses the session ID to refresh the displayed data and would error out if the session id is an empty string).
If sessionId is a nullable string in SkyFrosts data-model then we should adjust our local model accordingly and change usages of sessionId to handle the case of null explicitly.