RESP3: restore the concept of null array for RESP2 compat.

src/t_zset.c

@@ -3277,7 +3277,7 @@ void blockingGenericZpopCommand(client *c, int where) {
     /* If we are inside a MULTI/EXEC and the zset is empty the only thing
      * we can do is treating it as a timeout (even with timeout 0). */
     if (c->flags & CLIENT_MULTI) {
-        addReplyNull(c);
+        addReplyNullArray(c);
         return;
     }