do not publish live data for broken MPPT controllers

in case the user defines the respective pins, an MPPT controller will be
setup and the websocket would previously publish its invalid data even
if no data at all was received. this lead to using an invalid index and
to publish a useless card. instead, skip those controllers.

if the connection to a controller breaks, it will always present as a
card in the live view with the respective serial number and the last
known data.

src/WebApi_ws_vedirect_live.cpp

@@ -130,7 +130,10 @@ void WebApiWsVedirectLiveClass::generateJsonResponse(JsonVariant& root, bool ful
 
         VeDirectMpptController::spData_t &spMpptData = spOptMpptData.value();
 
-        const JsonObject &nested = array.createNestedObject(spMpptData->SER);
+        String serial(spMpptData->SER);
+        if (serial.isEmpty()) { continue; } // serial required as index
+
+        const JsonObject &nested = array.createNestedObject(serial);
         nested["data_age_ms"] = VictronMppt.getDataAgeMillis(idx);
         populateJson(nested, spMpptData);
         _lastPublish = millis();