Fix sub-second negative Duration serialization.

[vriesk]

This addresses the problem that the Duration's sign will "disappear" both during serialization and deserialization of sub-second Durations.

[codecov[bot]]

Codecov Report

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

Project coverage is 97.6%. Comparing base (4a74924) to head (e596d04). Report is 43 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #692 +/- ## ======================================= - Coverage 97.8% 97.6% -0.2% ======================================= Files 81 83 +2 Lines 9378 9010 -368 ======================================= - Hits 9169 8792 -377 - Misses 209 218 +9 ```

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