Merge conflicts message isn't displayed when diff_head param isn't passed

The project has default_merge_ref_for_diffs enabled. This merge request has conflicts: !22505 (diffs), master (base) version is displayed, but there's no message

One of the possible solutions would be to check here whether default_merge_ref_for_diffs FF is enabled: https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/diffs/components/app.vue

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