Grade empty numberlines as empty

Summary:
The validate function was defaulting `initialX` differently
than the renderer, causing the NumberLine to grade students
as wrong when they hadn't moved it, if `initialX` was unset
in the editor.

See line 193 or 831 for the other places this is defaulted
(yes, this should be refactored).

Test Plan:
Answer only the checkboxes in the following question, then
press score. Make sure you get back an `empty: true` response
and that the widget highlights in orange:
[test question](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22Plot%20the%20point%20corresponding%20to%20%24-%5C%5Cdfrac27%24%20on%20the%20number%20line%20below.%5Cn%5Cn%5Cn%5Cn**Which%20of%20the%20following%20inequalities%20are%20correct%3F**%5Cn%5Cn%5B%5B%E2%98%83%20radio%202%5D%5D%5Cn%5Cn%5Cn%5Cn%5B%5B%E2%98%83%20number-line%201%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22radio%202%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%22%20%24-%5C%5Cdfrac27%5C%5C%2C%5C%5C%2C%5C%5C%20%3C%5C%5C%2C-%5C%5Cdfrac37%24%22%2C%22correct%22%3Afalse%7D%2C%7B%22correct%22%3Atrue%2C%22content%22%3A%22%20%24-%5C%5Cdfrac37%5C%5C%2C%5C%5C%2C%5C%5C%20%3C%5C%5C%2C-%5C%5Cdfrac27%24%22%7D%2C%7B%22content%22%3A%22%20%20%24%5C%5C%2C%5C%5C%2C%5C%5C%2C%5C%5C%2C%5C%5C%2C%5C%5C%2C0%5C%5C%2C%5C%5C%2C%5C%5C%20%3E%5C%5C%2C%20-%5C%5Cdfrac37%24%22%2C%22correct%22%3Atrue%7D%5D%2C%22randomize%22%3Atrue%2C%22multipleSelect%22%3Atrue%2C%22displayCount%22%3Anull%7D%7D%2C%22number-line%201%22%3A%7B%22type%22%3A%22number-line%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22range%22%3A%5B-1%2C1%5D%2C%22labelStyle%22%3A%22improper%22%2C%22correctX%22%3A-0.2857142857142857%2C%22isTickCtrl%22%3Atrue%2C%22labelRange%22%3A%5B%22%22%2C%22%22%5D%2C%22divisionRange%22%3A%5B1%2C10%5D%2C%22labelTicks%22%3Atrue%2C%22numDivisions%22%3A5%2C%22tickStep%22%3Anull%2C%22snapDivisions%22%3A2%2C%22correctRel%22%3A%22eq%22%2C%22initialX%22%3Anull%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22Drag%20the%20point%20and%20select%20all%20correct%20answers.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%7B%22content%22%3A%22Let's%20look%20at%20the%20numbers%20on%20a%20number%20line.%20%20%20If%20we%20divide%20the%20line%20%20into%20sevenths%2C%20we%20can%20%20plot%20%24-%5C%5Cdfrac27%24%20%2C%20%20%24-%5C%5Cdfrac37%24%20%2C%20and%20their%20opposites.%5Cn%5Cn%5Cn%5Cn!%5B%5D(https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Ffe5187cabae037a9237c1e42a8c218d0039351e2.png)%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Ffe5187cabae037a9237c1e42a8c218d0039351e2.png%22%3A%7B%22width%22%3A460%2C%22height%22%3A80%7D%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22Since%20numbers%20to%20the%20left%20are%20less%20than%20numbers%20to%20the%20right%2C%20we%20have%20%5Cn%5Cn%24%5C%5Cqquad%20%20-%5C%5Cdfrac37%5C%5C%2C%5C%5C%2C%5C%5C%20%3C%5C%5C%2C-%5C%5Cdfrac27%24%5Cn%5Cnand%20%5Cn%5Cn%24%5C%5Cqquad%5C%5C%2C%5C%5C%2C%5C%5C%2C%5C%5C%2C%5C%5C%2C%5C%5C%2C%5C%5C%2C%5C%5C%2C0%5C%5C%2C%5C%5C%2C%5C%5C%20%3E%5C%5C%2C%20-%5C%5Cdfrac37%24%20.%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%7B%22content%22%3A%22We%20have%3A%20%20%5Cn!%5B%5D(https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F510f35988b27a3588aaa3c0b5bed8d6c9c34b7ff.png)%5Cn%5Cn*%20%24%20-%5C%5Cdfrac37%5C%5C%2C%5C%5C%2C%5C%5C%20%3C%5C%5C%2C-%5C%5Cdfrac27%24%5Cn%5Cn%5Cn*%20%20%24%5C%5C%2C%5C%5C%2C%5C%5C%2C%5C%5C%2C%5C%5C%2C%5C%5C%2C0%5C%5C%2C%5C%5C%2C%5C%5C%20%3E%5C%5C%2C%20-%5C%5Cdfrac37%24%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2F510f35988b27a3588aaa3c0b5bed8d6c9c34b7ff.png%22%3A%7B%22width%22%3A460%2C%22height%22%3A80%7D%7D%2C%22widgets%22%3A%7B%7D%7D%5D%7D)]

Reviewers: charlie

Reviewed By: charlie

Differential Revision: http://phabricator.khanacademy.org/D10626