`all_samples` in `full` protocol of utfvp dataset

Hi,

It seems that all_samples in full protocol of the utfvp dataset has a problem and raises an issue.

Here is a sample code:

from bob.bio.vein.database.utfvp import UtfvpDatabase
database = UtfvpDatabase(protocol="full")
database.all_samples()

and here is the error:

packages/bob/bio/base/database/csv_dataset.py", line 494, in all_samples
    samples = samples + self.background_model_samples()
  File "..../lib/python3.9/site-packages/bob/bio/base/database/csv_dataset.py", line 381, in background_model_samples
    self.csv_to_sample_loader.transform(self.train_csv)
  File "..../lib/python3.9/site-packages/sklearn/pipeline.py", line 635, in transform
    Xt = transform.transform(Xt)
  File "..../lib/python3.9/site-packages/bob/pipelines/sample_loaders.py", line 84, in transform
    X.seek(0)
AttributeError: 'NoneType' object has no attribute 'seek'