mqttClient allows calling connect without set clientName, yields out of memory error

[pljakobs]

mqttClient::connect uses copyString to copy the clientName into the protocol header. copyString tries

	destBuffer.data = (uint8_t*)MQTT_MALLOC(sourceString.length());

which, for a zero byte string will return, if I understand correctly, nullptr
(and also trigers an M:0 from the ESP)

	if(destBuffer.data == nullptr) {
		debug_e("Not enough memory");
		return false;
	}

will be true and the connect will fail.

I guess one way to avoid that is to guard against an empty clientName in mqttClient::connect. I'll have a better look and submit a pr if that is the only place this fails.

[pljakobs]

I think this should do it #2897