patch 8.0.0309: cannot use an empty key in json

Problem:    Cannot use an empty key in json.
Solution:   Allow for using an empty key.
diff --git a/src/json.c b/src/json.c
index fd1b6ec..e5e85eb 100644
--- a/src/json.c
+++ b/src/json.c
@@ -863,7 +863,7 @@
 		    && cur_item != NULL)
 	    {
 		top_item->jd_key = get_tv_string_buf_chk(cur_item, key_buf);
-		if (top_item->jd_key == NULL || *top_item->jd_key == NUL)
+		if (top_item->jd_key == NULL)
 		{
 		    clear_tv(cur_item);
 		    EMSG(_(e_invarg));