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
2 jobs from master in 7 minutes and 29 seconds
Status Job ID Name Coverage
  Source Check
passed #242847
eslint

00:00:29

 
  Build
passed #242848
flatpak

00:07:00