`common.schema.Transform.Sample()` silently discards `base_frame` about drake

[jwnimmer-tri]

Assuming that we don't want common.schema.Transform.Sample() to return common.schema.Transform, it should instead assert that base_frame is nullopt.

This would be fine by me. (But throw an error; don't literally assert.)

If we like, we could add a different function name to return a sampled copy of the Transform with the base name intact.

[jwnimmer-tri]

I would have thought that the same complaint about "silently ignoring base_frame" would have applied to GetDeterministicValue() as well?