[Feature Request] always output NMEA strings, when in autopilot mode, even without a valid fix

[kubark42]

Avare autopilot mode should always publish NMEA strings whenever it is connected to a serial device, not just publish data when there is a fix.

• NMEA strings support flags for when there is not a valid fix. Thus there is no risk to the autopilot.
• The autopilot should be able to detect a working hardware connection regardless of whether Avare has a lock.

Use case: Part of my preflight is to check that the autopilot is working. If I am preflighting inside a hangar, Avare does not have a lock. As a result of not sending NMEA data, I cannot complete my preflight until after I'm outside and have taxied into the clear.