Use lonely operator to simplify code

Remove changelog

app/views/projects/services/mattermost_slash_commands/_detailed_help.html.haml

-- pretty_name = defined?(@project) ? @project.full_name : _('<project name / alias>')
+- pretty_name = @project&.full_name || _('<project name / alias>')
 - run_actions_text = s_("ProjectService|Perform common operations on GitLab project: %{project_name}") % { project_name: pretty_name }
 
 %p= s_("ProjectService|To set up this service:")

app/views/projects/services/slack_slash_commands/_help.html.haml

-- pretty_name = defined?(@project) ? @project.full_name : _('<project name / alias>')
+- pretty_name = @project&.full_name || _('<project name / alias>')
 - run_actions_text = s_("ProjectService|Perform common operations on GitLab project: %{project_name}") % { project_name: pretty_name }
 
 .info-well

changelogs/unreleased/210528-fix-broken-links-in-some-instance-level-integrations.yml

----
-title: Fix broken links in some Instance-level integrations
-merge_request: 27298
-author:
-type: fixed