change "required" nodes to "not null"

[adamlodge]

Documenting some thinking about the true meaning of ""Required" when configuring individual nodes in a card/resource model.

It seems to me that what we really mean is that if you are going to save a tile of data from the given card, then this particular node cannot be null. I would suggest we modify the verbiage to "not null" rather than "required".

Part of what I like about this is that it is consistent with typical RDBMS verbiage. You are essentially setting a not null constraint on that node in the resource model.

This change would go hand in hand with #4375

[chiatt]

I like the idea of being consistent with RDBMS terms, but not null wouldn't be accurate because a geojson object with no features would still be 'required'. The same should be true for strings. You might store a string that has an object in the tile like so: {'en': ''}. It's technically not null, but you users should still be required to save a string value in order to save the card.