patch 8.2.1337: Vim9: cannot use empty key in dict assignment

Problem:    Vim9: cannot use empty key in dict assignment.
Solution:   Allow empty key. (closes #6591)
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 9bfec3a..de29988 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -1422,11 +1422,8 @@
 		    dict_T	*dict = tv_dict->vval.v_dict;
 		    dictitem_T	*di;
 
-		    if (key == NULL || *key == NUL)
-		    {
-			emsg(_(e_emptykey));
-			goto on_error;
-		    }
+		    if (key == NULL)
+			key = (char_u *)"";
 		    tv = STACK_TV_BOT(-3);
 		    di = dict_find(dict, key, -1);
 		    if (di != NULL)