Fix XREAD returning error on empty stream with + ID

When using XREAD STREAMS <stream> + on an empty stream created with MKSTREAM, valkey returns an error instead of nil.

This heppens because is missing a check on the stream length

The fix adds the lenght check on the condition

Fixes #2728

Signed-off-by: diego-ciciani01 <diego.ciciani@gmail.com>

src/t_stream.c

@@ -2275,7 +2275,7 @@ void xreadCommand(client *c) {
                                  "just return an empty result set.");
                 goto cleanup;
             }
-            if (o) {
+            if (o && ((stream *)o->ptr)->length) {
                 stream *s = o->ptr;
                 ids[id_idx] = s->last_id;
                 if (streamDecrID(&ids[id_idx]) != C_OK) {