Plane: AUTO mode VTOL takeoff is skipped if home is not set

If home is not set a VTOL takeoff will be skipped. This can happen if the vehicle is switched into auto too soon after boot. This bug could also result in taking off to the incorrect altitude if home is moved while the vehicle is already in auto mode.

This is the early return which bypasses the takeoff if the alt frame can't be converted. The conversion might need home.
https://github.com/ArduPilot/ardupilot/blob/be40f5985f608ef8baa7c653384a3ab3d3834a82/ArduPlane/quadplane.cpp#L3375-L3376

**Version**
What version was the issue encountered with

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

Start at ArduPlane/quadplane.cpp lines 3375-3376 and trace the altitude-frame conversion used by AUTO VTOL takeoff. Check behavior when home is unset at mode entry and when home moves; done means takeoff is not skipped and the vehicle does not use an incorrect altitude.

Written by the indexing model from the issue text.