Increase code coverage of DeployToken to 100%, and make sure all nullable fields have tests

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gitlab-org/release/retrospectives#1 (closed) describes an incident with deploy tokens that was partially caused by a lack of proper code coverage, and specifically data coverage as in this case a nullable column had no tests for when the value was null.

We should make sure that the code coverage of DeployToken is 100% in both CE and EE, and that any column that can be NULL has tests for that case.