Custom Domains for Pages cannot be disabled (bad yml output)

Even when in /etc/gitlab/gitlab.rb the external IP addresses for GitLab pages are disabled:

gitlab_pages['external_http'] = nil
gitlab_pages['external_https'] = nil

the "New Domain" button appears in GitLab.

The issue seems to be with the .inspect in the gitlab.yml.erb, where nil is inserted into the YAML instead of null or just nothing.

Package version is 9.4.3-ce.0; the problem did not happen with 8.17.7.