Fix triangle proofs empty instead wrong

Here are some bug fixes for this Trello card: https://trello.com/c/zrXh7J0Z/600-fix-triangle-proofs-exercises-to-grade-the-initial-selection-as-empty-rather-than-wrong

I also fixed a bug where the MathJax/Katex hints weren't being rendered correctly for Geometry Proofs Intro: Shared Problem type.

Geometry proofs 2 looked fine when I tested it, so no changes there.

A few notes on the the bug fix:

- For geometry proofs intro, there are some 2 step proofs. Once you fill out the first step, the next step appears. Currently if the user clicks "Check Answer" at this point, the answer will be marked as empty. Not sure if this was the behavior you wanted. My reasoning is that users could be confused and click "Check Answer" after finishing the first step. 
- For both geometry proofs 1 and intro, answers will be marked as empty if the user fills out some values and then clears out their answer.
- Should there be some kind of message displayed if the user clicks "Check Answer" without trying the problem?

Here are the test cases I used:

#### Test Cases for Geometry Proofs Intro

__Problem Type: Shared__
- Click "Check Answer". Should be marked as empty" (passed)
- Select value from select box and click "Check Answer". Should be marked as incorrect (passed)
- Unselect previous value, fill in one of the inputs, and click "Check Answer". Should be marked as incorrect (passed)
- Clear all values, click "Check Answer". Should be marked as empty" (passed)
- Get correct answer. Should be marked as correct (passed)

__Problem Type: Vertical and Alternate__
- Click "Check Answer". Should be marked as empty" (passed)
- Select value from select box and click "Check Answer". Should be marked as incorrect (passed)
- Unselect previous value, fill in one of the inputs, and click "Check Answer". Should be marked as incorrect (passed)
- Clear all values, click "Check Answer". Should be marked as empty (passed)
- Pass first part of proof, and click "Check Answer". Should be marked as empty. (passed)
- Select value from select box and click "Check Answer". Should be marked as incorrect (passed)
- Unselect previous value, fill in one of the inputs, and click "Check Answer". Should be marked as incorrect (passed)
- Get correct answer. Should be marked as correct (passed)

#### Test Cases for Geometry 1
- Click "Check Answer". Should be marked as empty" (passed)
- Select value from select box and click "Check Answer". Should be marked as incorrect (passed)
- Reset and fill input value from select box and click "Check Answer". Should be marked as incorrect (passed)
- Fill in one step of proof correctly. Click "Check Answer". Should be marked as incorrect (passed).
- Get correct answer. Should be marked as correct (passed)