Validation of `Result` should prevent `POST` insertion if `task` or `method` are `null`

[WrathfulSpatula]

We noticed with #131, inserting a Result with a null task or method field was not a problem. This led us to find the problem for #131, in the DTO for Submission.

However, we shouldn't be able to POST a new Result that has a null task or method. @vprusso Please invert the logical point of your unit test, and make sure that Result POST insertion validation rejects a request with a null task or method, thank you!