Add tests for passing nil values in epics and packages

In #215936 (closed), we found that upgrading to Grape v1.3.2 caused a regression in our QA suite because passing a nil value for an Array type would be passed in as a nil value instead of being coerced to an empty Array. We had to revert the upgrade as a result. To ensure that nil values are handled properly on the backend, add tests for this.