Takeoff - allow publishing NAN longitude and latitude

[bresch]

A NAN is interpreted in the FlightTaskAuto as a non-valid global coordinate and sets a local position.
If a zero is sent instead, the global coordinate is recognized as valid and will be executed.
This is a problem when the global position is gained for the first time after takeoff and that a valid global reference did not exist before

fixes #12919

To reproduce:

• Trigger an automatic takeoff without having a global position valid but optical flow only (ekf2 stop, gps stop, ekf2 start)
• Enable gps in air (gps start ...)
• The drone does not fly towards longitude/latitude 0 anymore

[jkflying]

To me this seems a major bugfix, since if we don't set them when the rep->current.lon is NaN, they remain 0.0, which is much more misleading/incorrect.

[bresch]

@PX4/testflights Could you test that PR please?
What's important here is to test autonomous takeoffs:

• start mission from ground
• takeoff triggered from ground station.

[jorge789]

Tested on CUAV+ V5:
Modes Tested

Mission Plan Mode (Automated): Good.
RTL: Good.

- Procedure

• Armed form QGC.
  Took-off on mission plan mode
  . Note that commands were sent form QGC (Armed and takeoff)
  Once all waypoint completed vehicle switched to RTL and landed as expected.
  Good flight in general.
  Notes:
  No issues noted, good flight in general.

Log:
https://review.px4.io/plot_app?log=de1e7c8b-b555-4706-b168-2d9a6d6bf4db

Tested on PixRacer V4:
Modes Tested

Mission Plan Mode (Automated): Good.
RTL: Good.

- Procedure

• Armed form QGC.
  Took-off on mission plan mode
  . Note that commands were sent form QGC (Armed and takeoff)
  Once all waypoint completed vehicle switched to RTL and landed as expected.
  Good flight in general.
  Notes:
  No issues noted, good flight in general.

Log:
https://review.px4.io/plot_app?log=0228f5f2-5470-4d2f-9a60-cfa1091b1366

[dannyfpv]

Tested on Pixhawk 4 f-450

Modes Tested
Mission Plan Mode (Automated): Good.
RTL: Good.

Procedure
Armed form QGC.
Took-off on mission plan mode
Note that commands were sent form QGC (Armed and takeoff)
Once all waypoint completed vehicle switched to RTL and landed as expected.
Good flight in general.

Notes:
No issues noted good flight in general.

Log:
https://review.px4.io/plot_app?log=0ca6226f-71c0-4774-b79b-00bc7986c79f

[bresch]

@jorge789 @dannyfpv Thanks guys, that's all good.