land_detector: publish when there is proper data
land_detector: publish when there is proper data

Instead of publishing before even having done update(), let's wait for a
result and then advertise.

Aerotenna/Firmware,PX4/Firmware,jlecoeur/Firmware,dagar/Firmware,PX4/Firmware,dagar/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,acfloria/Firmware,mcgill-robotics/Firmware,Aerotenna/Firmware,krbeverx/Firmware,Aerotenna/Firmware,mje-nz/PX4-Firmware,jlecoeur/Firmware,PX4/Firmware,dagar/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,darknight-007/Firmware,acfloria/Firmware,dagar/Firmware,darknight-007/Firmware,Aerotenna/Firmware,PX4/Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,darknight-007/Firmware,acfloria/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,acfloria/Firmware,krbeverx/Firmware,Aerotenna/Firmware,krbeverx/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,dagar/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,dagar/Firmware,jlecoeur/Firmware,jlecoeur/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,mcgill-robotics/Firmware,darknight-007/Firmware,krbeverx/Firmware,darknight-007/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,mcgill-robotics/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,jlecoeur/Firmware