fix for tier groups with empty tiers

[codecov[bot]]

Codecov Report

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

Project coverage is 91.34%. Comparing base (b018a8b) to head (142f07e). Report is 15 commits behind head on dev.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #203 +/- ## ======================================= Coverage 91.34% 91.34% ======================================= Files 15 15 Lines 1940 1940 ======================================= Hits 1772 1772 Misses 168 168 ```

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