Update VelocityTracker_test to Reflect Empty-Velocities

VelocityTracker can now return empty optional for velocities, in cases
where a given axis is not supported, or there is not enough data to
calculate velocity. This CL updates the test to reflect that capability
(came up during a code review for b/249171348).

Bug: 249171348
Test: unit tests pass
Change-Id: I6054450548d42d96af9957e55ea2c92790d1bb86