Service "External Wiki" only works when project Wiki is enabled

Summary

I have enabled the service "External Wiki" on a number of projects.
However, I see that the Wiki link substitution only works for projects that accidentally had their GitLab Wiki enabled. When the GitLab Wiki of a project is disabled, there is no Wiki link at all; hence no substitution.
This may be logical and consistent behaviour, but it is not what I would expect as a user.
Please let "External Wiki" show a link even if the GitLab Wiki is disabled.

Steps to Reproduce

• Create a new project
• Disable GitLab Wiki feature in project settings
• Enable "External Wiki" service for project

Expected Results

• Wiki link present on project page navigation
• Wiki link will direct to external wiki

Actual Results

• Wiki link is not visible

Additional Details

• Project exhibiting the behaviour: https://gitlab.com/issue-reproduce/external-wiki-gitlab-wiki-enabled

It seems that the ability class is responsible for this. "read_wiki" ability is unauthorised is the wiki project feature is disabled regardless of whether the external wiki is enabled.