Merge branch 'cat-fix-registry-tag-sizes-docs' into 'master'

Fixes the registry disk space usage script

See merge request gitlab-org/gitlab!24461

doc/administration/troubleshooting/gitlab_rails_cheat_sheet.md

@@ -941,7 +941,7 @@ projects.each do |p|
 
    container_repositories.each do |c|
        c.tags.each do |t|
-          project_total_size = project_total_size + t.total_size
+          project_total_size = project_total_size + t.total_size unless t.total_size.nil?
        end
    end