feat: units_of things without units returns None

[codecov[bot]]

Codecov Report

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

Project coverage is 95.84%. Comparing base (ca296a1) to head (7962c6b). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #260 +/- ## ======================================= Coverage 95.84% 95.84% ======================================= Files 42 42 Lines 1539 1541 +2 ======================================= + Hits 1475 1477 +2 Misses 64 64 ```

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