Catch/prevent null values in chargeLimits

[MikeBishop]

In debugging #451, it appears there's a way for the outside limit to be saved as null. The first commit should prevent any exceptions being thrown when that process encounters one that's already stored.

The second heads off the path that looks like it could store a null/None in a particular corner case: Vehicle reports ready, but updating location fails. Rather than storing the result and checking whether location was successful at multiple places, it just bails if the location couldn't be updated. It will run again shortly anyway.

There's also a surprisingly long path through the logic for a car that was and remains absent, and I'm not sure it doesn't update the data stored in settings. I added a short-circuit for that case, just to be safe.

[MikeBishop]

@ngardiner, these failures look unrelated to this change. Can I disregard the tests here and merge, or is there something that needs more attention?

[ngardiner]

Hi @MikeBishop - feel free to go ahead and merge anything you had queued despite these build errors, it looks like there's an error introduced with a recent dependency that was added, I'll need to work out why and to fix it