[radio] Display "None of the above" if no real answer is provided
[radio] Display "None of the above" if no real answer is provided

Summary:
If the user selects "None of the above" and that choice is correct,
but no real answer is provided, the radio widget will not replace
"None of the above" with "".

Test Plan:
Choose "none of the above" in [this
item](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20radio%201%5D%5D%5Cn%5Cn%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22radio%201%22%3A%7B%22type%22%3A%22radio%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22choices%22%3A%5B%7B%22content%22%3A%22A%22%2C%22correct%22%3Afalse%7D%2C%7B%22content%22%3A%22B%22%2C%22correct%22%3Afalse%7D%2C%7B%22isNoneOfTheAbove%22%3Atrue%2C%22correct%22%3Atrue%2C%22content%22%3A%22%22%7D%5D%2C%22randomize%22%3Afalse%2C%22multipleSelect%22%3Afalse%2C%22displayCount%22%3Anull%2C%22hasNoneOfTheAbove%22%3Atrue%2C%22onePerLine%22%3Atrue%2C%22deselectEnabled%22%3Afalse%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%5D%7D)
Press "score".
See "None of the above" still in the content.

Repeat but with a revealed answer provided; see that the answer is
revealed.

Auditors: emily

daukantas/perseus,alexristich/perseus,ariabuckles/perseus,sachgits/perseus,abe4mvp/perseus,learningequality/perseus,learningequality/perseus,daukantas/perseus,ariabuckles/perseus,chirilo/perseus,alexristich/perseus,iamchenxin/perseus,abe4mvp/perseus,chirilo/perseus,sachgits/perseus,ariabuckles/perseus,ariabuckles/perseus,iamchenxin/perseus