speechdispatcherfactory: Avoid setting voice name to None

When letting orca use the default speech dispatcher voice for a given language, the name would be None. We should however refrain from asking speech-dispatcher for a "None" voice since that'd make the output module try to fetch it and possibly fail and thus end up with broken language.

Reported by Didier Spaier on https://lists.gnu.org/archive/html/speechd-discuss/2022-09/msg00000.html