MUC: Identify subject change correctly

Fixes #9491
3 jobs for gajim_1.1 in 3 minutes and 32 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
passed #9206
run-appdata

00:00:10

passed #9205
run-tests

00:03:09

 
  Build
passed #9207
run-build

00:00:12