Navigation: Drone yaw hardcoded to zero if not given

[jnippula]

To make drone behaving properly and e.g. front camera pointing forward, there shall be a way to make drone heading pointing towards flying direction. So, there could be this kind of behaviour when giving waypoints to the navigation service:

1. If yaw value is given the drone shall use given yaw value in the waypoint (as currently implemented).
2. If yaw value is not given (float value NaN), the drone heading shall point forward, i.e. towards the flying direction.