patch 8.2.4683: verbose check with dict_find() to see if a key is present

Problem:    Verbose check with dict_find() to see if a key is present.
Solution:   Add dict_has_key(). (Yegappan Lakshmanan, closes #10074)
diff --git a/src/sign.c b/src/sign.c
index 3d9c387..673d902 100644
--- a/src/sign.c
+++ b/src/sign.c
@@ -2786,7 +2786,7 @@
 	    if (buf == NULL)
 		goto cleanup;
 	}
-	if (dict_find(dict, (char_u *)"id", -1) != NULL)
+	if (dict_has_key(dict, "id"))
 	{
 	    sign_id = dict_get_number(dict, (char_u *)"id");
 	    if (sign_id <= 0)