No components found when `--participant-label` requests only non-existant subjects

Per #216, `--participant-label` is supposed to apply after dataset parsing and querying. If only non-existent subjects are requested, a valid but empty component should be delivered. This is not currently happening. For performance reasons, the `--participant-label` filters are being applied via `layout.get` from pybids, but in the above case, return an empty list leading to datasets not being found.

Trace the `--participant-label` handling and its calls to `layout.get` in the dataset parsing and querying path. Reproduce the case where only non-existent subjects are requested, then verify that a valid empty component is delivered instead of treating the dataset as missing.

Written by the indexing model from the issue text.