core/frame: Fix ast_frdup() and ast_frisolate() for empty text frames

If a channel creates an AST_TEXT_FRAME with datalen == 0, the ast_frdup()
and ast_frisolate() functions could create a clone frame with an invalid
data.ptr which would cause a crash.  The proposed fix is to make sure that
for such empty text frames, ast_frdup() and ast_frisolate() return cloned
text frames with a valid data.ptr.

ASTERISK-28076
Reported by: Emmanuel BUU
Tested by: Emmanuel BUU

Change-Id: Ib882dd028598f13c4c233edbfdd7e54ad44a68e9