mainWindow: Avoid warning when no room was selected previously

On startup we first load the previously joined rooms, then select
the last selected one. However the setting for the latter may return
an empty object (when there were no rooms to restore), resulting in
a JS warning about accessing an undefined property.

Add a check to avoid that warning.

!100
4 jobs for wip/fmuellner/fix-js-warning in 7 minutes and 26 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Source Check
passed #242833
eslint

00:00:30

 
  Build
passed #242834
flatpak

00:06:23

 
  Review
passed #242835
review

00:00:31

manual #242836
allowed to fail manual
stop_review