Review result-filtration discomfort re: `add_columns`

[abradyGDIT]

for subject CCDI.PAUIAV, which has no associated files:

• get_subject_data( match_all=[ 'subject_id = CCDI.PAUIAV' ] ) produces one record, as expected
• get_subject_data( match_all=[ 'subject_id = CCDI.PAUIAV', 'anatomic_site = null' ] ) produces zero records, which may be problematic
• for any file_column in the file table:
  • get_subject_data( match_all=[ 'subject_id = CCDI.PAUIAV' ], add_columns='file_column' ) produces zero records, which is probably problematic