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/map.c b/src/map.c
index f3dd569..917f1a2 100644
--- a/src/map.c
+++ b/src/map.c
@@ -2475,7 +2475,7 @@
     char_u	*s;
     char_u	*cpo_save = p_cpo;
 
-    p_cpo = (char_u *)"";	// Allow <> notation
+    p_cpo = empty_option;	// Allow <> notation
     s = vim_strsave(map);
     if (s != NULL)
     {