display null, undefined, "" in Applab Data Table

[ajpal]

Description

Currently all these values display as blank, but we want to show them separately so that students can tell what's actually in their table. This will also be important for the data visualizer work, which allows students to filter by specific value. For this, they will be able to be filter separately by "", null, or undefined, so it's important that they be able to tell what the actual value is in the table view.

Before:

After:

This change also applies to key/value pairs (undefined is not possible here)

Testing story

Reviewer Checklist:

• Tests provide adequate coverage
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked

[epeach]

I think JSON.Stringify('') returns "''" (hard to see that but it quotes the quote) - is that intended here?

[ajpal]

For now, I think yes. Lots of open questions still about how to correctly parse user input.
But just to keep things moving for dataviz, I think we can punt on some of those questions for now.