can't enforce presence of array elements

Neither of the GraphQL type decorations can enforce that an array should have some items:
- `[Type!]!` means a mandatory array of non-null items, but there can be 0 items
- `[Type]!` means a mandatory array of items that can be null or missing altogether
- `[Type!]` means an optional array of non-null items, and there can be 0 items

So there's no way to say "this multi-valued field must have at least one value"

https://www.apollographql.com/blog/using-nullability-in-graphql-2254f84c4ed7 by @stubailo puts it like this: "One interesting conclusion here is that there’s no way to specify that the list can’t be empty — an empty list `[]` is always valid, regardless of whether the list or items are non-null."

Interesting indeed; but I think this really is a failure of the spec to capture this basic requirement. You can control whether array elements are non-null, which is not a common requirement (and given #719, you should never use); but you cannot state there must be some elements, which is a very common requirement.