[v2.11] Fix meta step down panic on empty object (#5993)

Side effect from #5956 some
clients/natscli send empty objects to trigger the meta leader step down.

server/jetstream_api.go

@@ -2833,7 +2833,7 @@ func (s *Server) jsLeaderStepDownRequest(sub *subscription, c *client, _ *Accoun
 	var preferredLeader string
 	var resp = JSApiLeaderStepDownResponse{ApiResponse: ApiResponse{Type: JSApiLeaderStepDownResponseType}}
 
-	if isJSONObjectOrArray(msg) {
+	if !isEmptyRequest(msg) {
 		var req JSApiLeaderStepdownRequest
 		if err := json.Unmarshal(msg, &req); err != nil {
 			resp.Error = NewJSInvalidJSONError(err)

server/jetstream_test.go

@@ -6712,6 +6712,21 @@ func TestJetStreamSystemLimitsPlacement(t *testing.T) {
 		t.Fatalf("unexpected memory stream create success, maxBytes=%d, replicas=%d",
 			si.Config.MaxBytes, si.Config.Replicas)
 	}
+
+	t.Run("meta info placement in request", func(t *testing.T) {
+		nc, err = nats.Connect(largeSrv.ClientURL(), nats.UserInfo("admin", "s3cr3t!"))
+		require_NoError(t, err)
+		defer nc.Close()
+
+		var resp JSApiLeaderStepDownResponse
+		ncResp, err := nc.Request(JSApiLeaderStepDown, []byte("{}"), 3*time.Second)
+		require_NoError(t, err)
+		err = json.Unmarshal(ncResp.Data, &resp)
+		require_NoError(t, err)
+		require_True(t, resp.Error == nil)
+		require_True(t, resp.Success)
+
+	})
 }
 
 func TestJetStreamStreamLimitUpdate(t *testing.T) {