fix(carriers): fix errors in another postnl carrier configuration

[EdieLemoine]

• occurs when no custom postnl contract is present but there are still multiple enabled postnl carriers returned from the api

[codecov[bot]]

Codecov Report

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

Project coverage is 96.22%. Comparing base (62220cf) to head (693d721).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #292 +/- ## ========================================= Coverage 96.22% 96.22% + Complexity 1665 1664 -1 ========================================= Files 307 307 Lines 5508 5509 +1 ========================================= + Hits 5300 5301 +1 Misses 208 208 ```

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