[google_maps_flutter] Fix handling of non-nullable invokeMethod return types

[stuartmorgan-g]

During the null-safety migration I accepted the auto-migrator use of as Future<T> to handle invokeMethod<T> returning a T?. I didn't realize that as does not actually do that kind of casting, and will fail with type 'Future<T?>' is not a subtype of type 'Future<T>' in type cast.

There were no tests that exercised these methods in any way, so automated tests didn't catch the bug. This adds a minimal test that calls all of the non-void methods to ensure that they don't explode (and a TODO to backfill full unit tests of the entire method channel).

Fixes flutter/flutter#78426
Fixes flutter/flutter#78856

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[cyanglaz]

LGTM