bonjour-self: Avoid removing msg from record if it's missing
bonjour-self: Avoid removing msg from record if it's missing

Avoid removing msg value from the TXT record if the key doesn't
exisit. Trying to do so results in KDNSService_NoSuchKey error,
and it fails the set_presence operation. which leads to many complications.

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