Settings API, `enabled_git_access_protocol` not accepting `nil` setting

Summary

Setting enabled_git_access_protocol to nil via the application settings API endpoint fails despite the API documentation stating that ssh, http and nil are accepted values for the setting.

Steps to reproduce

Make the API call below to set enabled_git_access_protocol to nil;

curl --silent --insecure -X PUT --header 'PRIVATE-TOKEN: <token>' --header 'Accept:application/json' 'https://<gitlab-url>/api/v4/application/settings?enabled_git_access_protocol=nil'

The command above outputs: {"message":{"enabled_git_access_protocol":["is not included in the list"]}}

Customer: https://gitlab.zendesk.com/agent/tickets/126859 (ZD Internal)