fix: Feedback addressed by adding a testing the edge case in plot_missing_vals

[jbarns14]

Fixes #81

[codecov[bot]]

Codecov Report

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

Comparison is base (37ab96c) 98.43% compared to head (853e2b7) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #100 +/- ## =========================================== + Coverage 98.43% 100.00% +1.56% =========================================== Files 5 5 Lines 64 64 =========================================== + Hits 63 64 +1 + Misses 1 0 -1 ```

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