avoid sending null through live data websockets

on the "main" live data websocket this can happen if no
OnBattery-specific data is available to sent but the empty
JSON document is still sent, which trips the web application.

publishing null in the battery live data websocket also trips the web
application, which rightfully assumes a valid object if data is received
through the websocket.

src/WebApi_ws_battery.cpp

@@ -67,6 +67,9 @@ void WebApiWsBatteryLiveClass::sendDataTaskCb()
             JsonVariant var = root;
             generateJsonResponse(var);
 
+            // battery provider does not generate a card, e.g., MQTT provider
+            if (root.isNull()) { return; }
+
             String buffer;
             serializeJson(root, buffer);
 

src/WebApi_ws_live.cpp

@@ -121,6 +121,8 @@ void WebApiWsLiveClass::sendOnBatteryStats()
     if (all) { _lastPublishOnBatteryFull = millis(); }
     generateOnBatteryJsonResponse(var, all);
 
+    if (root.isNull()) { return; }
+
     String buffer;
     serializeJson(root, buffer);