Don't create empty literal pieces

Don't create empty literal pieces

I noticed that format_pieces can create an empty literal piece.
However, there's never a need for one, so this patch removes the
possibility.

gdb/ChangeLog
2019-10-01  Tom Tromey  <tom@tromey.com>

	* unittests/format_pieces-selftests.c: Update.  Add final format.
	* gdbsupport/format.c (format_pieces::format_pieces): Don't add
	empty literal pieces.