fix: allow null label colors in trello json importer

[jacksbox]

• Resolves: Trello import: null values in label colors #5353
• Target version: main

Summary

Sometimes the exported json from trello contains null values for labels. This causes an error on import
This is caused by the translateColor method, which expects a string.
The PR solves this by making the passed in $color optional, and falling back to using the default color.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Documentation (manuals or wiki) has been updated or is not required

[juliusknorr]

Thanks for the pull request.

[juliusknorr]

/backport to stable28

[juliusknorr]

/backport to stable27

[juliusknorr]

/backport to stable26

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