[BRID] choice_track() loads tracks without annotations

[giovana-morais]

BRID has two kinds of data: mixtures and solo tracks but we only have beat annotations for the mixtures. when we tried to load a random track to validate the annotations, the code broke because it was trying to get a track without the annotations

steps to reproduce:

data_home = os.path.join(PATH, 'mir_datasets', 'brid')
brid = mirdata.initialize('brid', data_home=data_home)
brid.download()
ct = brid.choice_track()
print(ct.track_id)
# "[0349] S3-TT1-02-PA"
print(ct.beats.times)
AttributeError: 'NoneType' object has no attribute 'times'

it is useful to provide all the tracks because people can work on other stuff than beat tracking, but maybe we should have a way of asking from a random track with annotations or general? something like
ct = brid.choice_track(with_annotation=True)?

cc: @magdalenafuentes