Don't crash when a GraphQL-requested workspace (with the random argument) doesn't exist

[caiosba]

Description

Don't crash when a GraphQL-requested workspace (with the random argument) doesn't exist.

Reported by Sentry.

Fixes: CV2-6151.

How has this been tested?

TDD. Reproduced in a unit test that passed after the fix:

Failure:
GraphqlController11Test#test_should_not_crash_if_workspace_doesn't_exist [test/controllers/graphql_controller_11_test.rb:397]:
Expected response to be a <2XX: success>, but was a <400: Bad Request>
Response body: {"errors":[{"message":"undefined method `reload' for nil:NilClass","code":5,"data":{}}]}

Checklist

• I have performed a self-review of my own code
• I have added unit and feature tests, if the PR implements a new feature or otherwise would benefit from additional testing
• I have added regression tests, if the PR fixes a bug
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• My changes generate no new warnings
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