patch 8.2.2289: Vim9: 'cpo' can become empty

Problem:    Vim9: 'cpo' can become empty.
Solution:   Use empty_option instead of an empty string.  Update quickfix
            buffer after restoring 'cpo'.  (closes #7608)

src/gui_motif.c

@@ -3525,7 +3525,7 @@ find_replace_callback(
 	char_u	*save_cpo = p_cpo;
 
 	// No need to be Vi compatible here.
-	p_cpo = (char_u *)"";
+	p_cpo = empty_options;
 	u_undo(1);
 	p_cpo = save_cpo;
 	gui_update_screen();