Resolve "GraphQL: getting MR diff discussions often returns 500"

What does this MR do?

A small MVC for: #298827 (closed)

When we ask GraphQL for discussions (including position) on an MR where the source project is a fork, the API call fails with 500 Internal Server Error.

Screenshots (strongly suggested)

N/A

Does this MR meet the acceptance criteria?

Conformity

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• Documentation (if required)
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• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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Related to #298827 (closed)