fix: Cannot name NULL values

[codecov-commenter]

Codecov Report

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

Comparison is base (3cc0a99) 93.59% compared to head (7aa95fb) 93.58%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #123 +/- ## ========================================== - Coverage 93.59% 93.58% -0.01% ========================================== Files 59 59 Lines 1483 1482 -1 ========================================== - Hits 1388 1387 -1 Misses 95 95 ``` | [Files](https://app.codecov.io/gh/nathaneastwood/poorman/pull/123?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Nathan+Eastwood) | Coverage Δ | | |---|---|---| | [R/mutate.R](https://app.codecov.io/gh/nathaneastwood/poorman/pull/123?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Nathan+Eastwood#diff-Ui9tdXRhdGUuUg==) | `100.00% <100.00%> (ø)` | |

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