Avoid warning with empty upper triangular solves

[codecov[bot]]

Codecov Report

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

Project coverage is 89.60%. Comparing base (8618b18) to head (eea7224). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #442 +/- ## ======================================= Coverage 89.60% 89.60% ======================================= Files 25 25 Lines 3569 3570 +1 ======================================= + Hits 3198 3199 +1 Misses 371 371 ```

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