DataDog integration settings can't be reverted from Custom to Default.

Summary

Trying to save the Datadog integration settings when a parent project has been configured for integration will fail to save with an error:

Steps to reproduce

1. Configure DataDog integration under Settings → Integrations → DataDog on a namespace with at least one existing project.
2. Set the existing project to Use Custom Settings with any values (valid or not) in the API URL and API Key fields
3. Save
4. Select Use Default Settings and save.

Example Project

https://gitlab.com/gitlab-silver/datadog-test/

What is the current bug behavior?

Unable to revert integration to use Default settings

What is the expected correct behavior?

Able to apply Default settings

Relevant logs and/or screenshots

Output of checks

This bug happens on Gitlab.com: GitLab Enterprise Edition 14.1.0-pre 4d9b43f1227

Possible fixes