test-muc-invite.py: avoid implicit assertion that no non-Text channels appear
test-muc-invite.py: avoid implicit assertion that no non-Text channels appear

This failed at least once for me while on an offline laptop.

freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut,freedesktop-unofficial-mirror/telepathy__telepathy-salut