Merge branch 'dm-empty-tree-readme' into 'master'

Don't blow up when a tree readme is empty and consequently has no rich viewer

Closes #32396

See merge request !11459

app/views/projects/tree/_readme.html.haml

-%article.file-holder.readme-holder
+- if readme.rich_viewer
+  %article.file-holder.readme-holder
     .js-file-title.file-title
       = blob_icon readme.mode, readme.name
       = link_to namespace_project_blob_path(@project.namespace, @project, tree_join(@ref, readme.path)) do