Fix CLUSTER SHARDS command returns empty array (#13422)

Close https://github.com/redis/redis/issues/13414

When the cluster's master node fails and is switched to another node,
the first node in the shard node list (the old master) is no longer
valid.
Add a new method clusterGetMasterFromShard() to obtain the current
master.

tests/cluster/tests/28-cluster-shards.tcl

@@ -124,6 +124,10 @@ test "Verify health as fail for killed node" {
     }
 }
 
+test "Verify that other nodes can correctly output the new master's slots" {
+    assert_not_equal {} [dict get [get_node_info_from_shard [R 4 CLUSTER MYID] 8 "shard"] slots]
+}
+
 set primary_id 4
 set replica_id 0