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)
diff --git a/src/syntax.c b/src/syntax.c
index 890158c..d4ec0d3 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -5670,7 +5670,7 @@
 
     // Make 'cpoptions' empty, to avoid the 'l' flag
     cpo_save = p_cpo;
-    p_cpo = (char_u *)"";
+    p_cpo = empty_option;
     ci->sp_prog = vim_regcomp(ci->sp_pattern, RE_MAGIC);
     p_cpo = cpo_save;
 
@@ -5858,7 +5858,7 @@
 
 		// Make 'cpoptions' empty, to avoid the 'l' flag
 		cpo_save = p_cpo;
-		p_cpo = (char_u *)"";
+		p_cpo = empty_option;
 		curwin->w_s->b_syn_linecont_prog =
 		       vim_regcomp(curwin->w_s->b_syn_linecont_pat, RE_MAGIC);
 		p_cpo = cpo_save;