sample_set.py extract results_files exclude None data_file

mostly a failsafe in case of bad data. In a normal environment this
should not matter.

See #3041

server/web2py/applications/vidjil/controllers/sample_set.py

@@ -290,6 +290,7 @@ def result_files():
             (db.sample_set.id.belongs(sample_set_ids)) &
             (db.sample_set_membership.sample_set_id == db.sample_set.id) &
             (db.results_file.sequence_file_id == db.sample_set_membership.sequence_file_id) &
+            (db.results_file.data_file != None) &
             config_query
         )