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/ex_eval.c b/src/ex_eval.c
index 2064cfa..94338fc 100644
--- a/src/ex_eval.c
+++ b/src/ex_eval.c
@@ -1695,7 +1695,7 @@
 		    *end = NUL;
 		}
 		save_cpo  = p_cpo;
-		p_cpo = (char_u *)"";
+		p_cpo = empty_option;
 		// Disable error messages, it will make current_exception
 		// invalid.
 		++emsg_off;