Python: Avoid exception on unsigned raster with negative nodata value

[codecov[bot]]

Codecov Report

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

Project coverage is 90.49%. Comparing base (5cfeff2) to head (85c1450). Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #130 +/- ## ========================================== - Coverage 90.52% 90.49% -0.04% ========================================== Files 85 85 Lines 6082 6083 +1 Branches 611 613 +2 ========================================== - Hits 5506 5505 -1 - Misses 544 546 +2 Partials 32 32 ```

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