app_voicemail: Prevent crash when saving message with realtime voicemail

ast_store_realtime() is not NULL tolerant, so we need to initialize
the field values we pass to it to the empty string to avoid a crash.

ASTERISK-23739 #close
Reported by: Stas Kobzar

Change-Id: I756c5dd0299c77f4274368f7c99eb0464367466c