ADS-B Squawk Codes are Always Zero

#### Issue details
Squawk Codes from ADS-B always show as 0.

I am receiving ADS-B data in SBS-1 format from a dump1090 receiver. This is essentially a type of CSV. Not every message will have the current squawk code of the aircraft and the squawk field will be left empty. Mission Planner is interpretting an empty field as a squawk code of "0".

Ideally it should remember the most recent non-null value and time out the value after a period of time such as 30 seconds.

A null squawk is different than a squawk code of zero.

#### Version
1.3.81 (build 1.3.8741.25556)

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
_What type of airframe (flying wing, glider, hex, Y6, octa etc)_

#### Hardware type
_What autopilot hardware was used? (pixhawk, pixracer, PX4FMU etc)_

#### Logs
_Please provide a link to any relevant logs that show the issue_

No source file or test is named. Reproduce an SBS-1 stream from dump1090 containing both populated and empty squawk fields, then trace how Mission Planner parses and displays those fields. Done means empty fields no longer overwrite a recent non-null squawk, and the retained value expires after the agreed timeout while an actual zero remains distinct.

Written by the indexing model from the issue text.