Allow testers to submit an empty output field when test is marked as having "No output"

[s3ththompson]

The test renderer currently requires text in the output field before a test can be submitted. When the AT did not output any speech, there is no text to be input (and the correct answer to the test is to check the "no output" box).

The UI should either make the field optional, provide some kind of validation based on whether "no output" was checked, or perhaps conditionally disable the field based on what is checked.