Migrate LatLngBounds to nullsafety

[TesteurManiak]

BREAKING CHANGES

Updated LatLngBounds constructor and its methods to rely on non-nullable values.

Migration guide

BAD:

final bounds = LatLngBounds();
bounds.extend(dublin);
bounds.extend(paris);
bounds.extend(london);

GOOD:

final bounds = LatLngBounds.fromPoints([dublin, paris, london]);

And all method related to the LatLngBounds now relies on non-nullable parameters and returns non-nullable values:

• LatLngBounds: takes 2 LatLng parameters instead of LatLng?
• extend(): takes a LatLng instead of LatLng?
• southWest: returns a LatLng instead of LatLng?
• northEast: returns a LatLng instead of LatLng?
• isValid: became deprecated as it'll always be true
• contains(): takes a LatLng instead of LatLng?
• isOverlapping(): takes a LatLngBounds instead of LatLngBounds?

I've added some unit tests to ensure that the values are still relevant even after the migration.

[JaffaKetchup]

LGTM. Thanks :)