When covariate missing, fall back to unadjusted inferences

[codecov-commenter]

Codecov Report

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

Project coverage is 83.96%. Comparing base (208ad7e) to head (70b35c7).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #268 +/- ## ========================================== + Coverage 83.05% 83.96% +0.91% ========================================== Files 18 18 Lines 1275 1285 +10 ========================================== + Hits 1059 1079 +20 + Misses 216 206 -10 ``` | [Flag](https://app.codecov.io/gh/mozilla/mozanalysis/pull/268/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla) | Coverage Δ | | |---|---|---| | [project](https://app.codecov.io/gh/mozilla/mozanalysis/pull/268/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla) | `83.96% <100.00%> (+0.91%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla#carryforward-flags-in-the-pull-request-comment) to find out more.

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