This is consistent with the other ff_ass_split_* functions.
It also fixes a crash when trying to split a dialog with text=NULL
(which seems to happen when the text of the dialog is empty); basically,
this commit fixes crashes when trying to encode an empty text subtitle
dialog (see subrip and mov_text encoders).
Fixes Ticket2048.
(cherry picked from commit c83002a4f8042ccfa0688a9a18e8fa0369c1fda8)
