More accurately determine if MQTT uses the default server

[esev]

This avoids needing to use strcmp each time a packet is sent as well.

Tested with the following values for moduleConfig.mqtt.address:

• An empty string (isDefaultMqttServer=true)
• mqtt.meshtastic.org (isDefaultMqttServer=true)
• mqtt.meshtastic.org:1883 (isDefaultMqttServer=true)
• 192.168.1.10:1883 (isDefaultMqttServer=false)

[fifieldt]

Is it worth also adding this to Channels.cpp ( Channels::anyMqttEnabled() ) to keep it consistent across the code base?

[esev]

Nice catch! It makes sense to me.

I checked all the callers of Channels::anyMqttEnabled(), and they each checked to see that mqtt was valid too (or the caller was MQTT). So I don't expect the behavior to change by adding the same mqtt && check inside Channels::anyMqttEnabled().