Broken sidebar icons in GitLab Branch Source plugin (regression in 2.341)

Since 2.341, the sidebar icons of the gitlab branch source plugin are broken:

Restoring to 2.340 and below resolves the problem.
To reproduce:

1. Create a new multibranch pipeline (workflow-multibranch-plugin)
2. Branch Sources -> Add Source -> GitLab Project (make sure you have the gitlab branch source plugin installed)
3.  In the "Owner" field add your GitLab username and the project you want to add in the "Projects" dropdown. Preferably you have a repository with a dummy Jenkinsfile, e.g. https://gitlab.com/NotMyFault/jenkinsfile to utilize the option "Jenkinsfile" on Build Configuration -> Script Path
4. Save the configuration, wait until the scanning is done and you can see the broken image on the sidebar of the job overview.

This issue is a copy of https://github.com/jenkinsci/gitlab-branch-source-plugin/issues/185 to track it on the ux regression dashboard on Jira.