Copter: waiting for terrain data if mission has no lat,lon defined

[rmackay9]

Bug Report

Copter allows some mission commands to provide no lat,lon. This seems to be tricking our terrain library and/or arming checks into reporting, "waiting for terrain data" indefinitely.

The issue can be reproduced in SITL by doing the following:

1. start Copter
2. create a mission with just a Takeoff to 20m (above terrain) and (optionally) a RETURN_TO_LAUNCH command

PreArm: waiting for terrain data will be displayed but note that "ter_pend" is zero (see below)

Version
4.7.0 and higher

Platform
[ ] All
[ ] AntennaTracker
[x] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

Airframe type
Copter

Hardware type
SITL, CubeOrange