Make i18n translate page work again with widgets

Summary: Occasionally we have a widget reference but no widget props, in which case we use the default props. This avoids indexing into the undefined props.widgets. It won't properly decide whether to highlight in this case, but that should be okay.

Test Plan:
Went to

  http://127.0.0.1:8080/translate/perseus?content=Abby+poured+herself+%24%5Cdfrac78%24+of+a+glass+of+juice.%0A%0A%24%5Cqquad%24!%5B%5D(https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fe19225e6aee73050b9c5bf936fdb71664813646f.png)+++%0A%0AHer+brother+Max+filled+a+different+glass+%24%5Cdfrac78%24+of+the+way+with+juice.%0A%0A**Who+had+more+juice+to+drink%3F**%0A%5B%5B%E2%98%83+radio+1%5D%5D

and saw it render, which it didn't do before.

Reviewers: justinjaffray, jack

Reviewed By: jack

Subscribers: alpert, james

Differential Revision: http://phabricator.khanacademy.org/D7939