🐛 [BUG] Fix error on APIv2 for Signages with pictograms (refs #3839)

[Chatewgne]

Description

Fix Line serializer in APIv2

Related Issue

Checklist

• [x] I have followed the guidelines in our Contributing document
• [x] My code respects the Definition of done available in the Development section of the documentation
• [x] I have performed a self-review of my code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works.
• [x] New and existing unit tests pass locally with my changes
• [x] I added an entry in the changelog file
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[codecov[bot]]

Codecov Report

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

Project coverage is 98.38%. Comparing base (0fea958) to head (bda7f04).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4254 +/- ## ======================================= Coverage 98.38% 98.38% ======================================= Files 267 267 Lines 20823 20827 +4 ======================================= + Hits 20486 20490 +4 Misses 337 337 ```

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[cypress[bot]]

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Test summary

22 • 0 • 0 • 0

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Run details

Project	Geotrek-admin
Status	Passed
Commit	5c65f17394 ℹ️
Started	Aug 2, 2024 12:53 PM
Ended	Aug 2, 2024 12:55 PM
Duration	02:01 💡
OS	Linux Ubuntu - 20.04
Browser	Electron 94

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