Numeric input answer close

I couldn't find a way to update my previous pull request so I just created a new one. I think I addressed all your comments, but I would love more feedback on how to improve!
What's the note about? I don't see `serialize` mentioned anywhere else in this diff.

Also, we probably shouldn't just remove blank answers here. That indicates an issue with the problem so if there are any blank answers that should be mentioned in `getSaveWarnings`.