talloc_string_sub2: Don't return NULL if src is empty.

This fixes BUG #5635.

Finished print jobs were not removed from the $PRINTER.tdb file if
"printing=cups".

In print_queue_update, talloc_string_sub2 is used to assemble the
"lprm command". In the case of using "printing=cups", the default
"lprm command" is an empty string. talloc_string_sub2 is called with
this empty string and returns NULL which leads to exiting
print_queue_update without doing the actual print queue update.

Signed-off by Michael Adam <obnox@samba.org>