Update timeline.js

[Phpvarious]

Ajout d'un chaînage optionnel (optional chaining).

Description

Lors de l'affichage de la timeline pour un profil user, les 2 boutons sont masqués dans le php, donc le addEventListener émet une erreur TypeError: document.getElementById(...) is null

Suggested changelog entry

Correction d’une erreur js lors de l'affichage de la timeline pour un profil "utilisateur"

Related issues/external references

Types of changes

• Bug fix (non-breaking change which fixes)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
  • This change is only breaking for integrators, not for external standards or end-users.
• Documentation improvement

PR checklist

• I have checked there is no other PR open for the same change.
• I have read the [La ligne directrice pour contribuer à ce projet / Contribution guidelines for this project).
• I grant the project the right to include and distribute the code under the GNU.
• I have added tests to cover my changes.
• I have verified that the code complies with the projects coding standards.
• [Required for new sniffs] I have added MD documentation for the sniff.