[8.13](backport #38962) x-pack/filebeat/input/{cel,httpjson}: fix oauth2 config validation

[mergify]

Proposed commit message

The logic for validation assumed that client.id and client.secret must be present, but this is not the case for password grant, so relax the requirement.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

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Related issues

• Ref elastic/integrations#9600

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This is an automatic backport of pull request #38962 done by [Mergify](https://mergify.com).

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