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
5 jobs for !100 with wip/fmuellner/fix-js-warning in 7 minutes and 59 seconds (queued for 2 seconds)
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Status Job ID Name Coverage
  Commit Check
passed #242837
check_commit_log

00:00:20

 
  Source Check
passed #242838
eslint

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  Build
passed #242839
flatpak

00:06:12

 
  Review
passed #242840
review

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passed #242841
allowed to fail manual
stop_review

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