SCHEMA: Update existence checks to consider empty lists

[effigies]

Fixes two issues found in real datasets:

1) stim_file columns with all n/a. This is already handled in a duplicated check: Guessing this got rebased out before pushing.

2) IntendedFor fields with an empty array ([]) as a value. Other reference checks suffer the same potential bug, so fixed at the same time.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.93%. Comparing base (bd08602) to head (f11fb28). Report is 43 commits behind head on master.

:exclamation: Current head f11fb28 differs from pull request most recent head 9467f62. Consider uploading reports for the commit 9467f62 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1747 +/- ## ======================================= Coverage 87.93% 87.93% ======================================= Files 16 16 Lines 1351 1351 ======================================= Hits 1188 1188 Misses 163 163 ```

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