Fix 500 error for non-existing snippet on graphql mutations

What does this MR do?

Fixes Issue: #214423 (closed)

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
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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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• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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