Fix crash when cleaning positions, minor improvements

Summary:
Avoid flipping direction if the bearing changes suddenly to null

Fix T1365

Test Plan:
Open map. See buses going around.

Put app in background. Re-open map. See buses still going around.

Open lines. Open a line. See buses going around.

Put app in background. Still see buses going around.

Tap bus stop. See arrivals.

Nothing explodes.

Reviewers: valerio.bozzolan

Reviewed By: valerio.bozzolan

Subscribers: valerio.bozzolan, Libre BusTO

Tags: Libre BusTO

Maniphest Tasks: T1365

Differential Revision: https://gitpull.it/D207