API requests for project push rules fails when using `NAMESPACE/PROJECT_NAME`

Summary

API requests for a project's push rules fails with a 404 error when the request is made using NAMESPACE/PROJECT_NAME but it succeeds when using the project's ID.

Steps to reproduce

(How one can reproduce the issue - this is very important)

Example Project

https://gitlab.com/ckatanda/api-push-rules-test

What is the current bug behavior?

Sending a GET request to https://gitlab.com/api/v4/projects/ckatanda/api-push-rules-test/push_rule?private_token=XXXXXXXX returns:

{
   "error": "404 Not Found"
}

What is the expected correct behavior?

The documentation mentions that one can use the project ID or NAMESPACE/PROJECT_NAME when making an API call for getting a project's push rules. Expected output is similar to what using the ID as https://gitlab.com/api/v4/projects/15093746 /push_rule?private_token=XXXXXXXX returns:

{
  "id": 7187570,
  "project_id": 15093746,
  "created_at": "2019-10-30T15:00:09.541Z",
  "commit_message_regex": "",
  "commit_message_negative_regex": "",
  "branch_name_regex": "",
  "deny_delete_tag": true,
  "member_check": true,
  "prevent_secrets": false,
  "author_email_regex": "",
  "file_name_regex": "",
  "max_file_size": 0,
  "commit_committer_check": null,
  "reject_unsigned_commits": null
}

This happens on self managed GitLab and on GitLab.com

Customer ticket: https://gitlab.zendesk.com/agent/tickets/137164 (Internal access only)