McdConnection: get_all_requests_cb: don't segfault if Channels property doesn't exist
McdConnection: get_all_requests_cb: don't segfault if Channels property doesn't exist

However buggy the other process is, we shouldn't crash.

freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control,freedesktop-unofficial-mirror/telepathy__telepathy-mission-control