Use `minItems` or lack to determine optionality for array items

[brettz9]

• Breaking enhancement: expect minItems for required array items; default to array items being optional

The prior behavior was that array items would all be marked as required (note that the JSON Schema required property expects property names, as with object names, not array indexes, so it can't be used with array items).

But the behavior with this PR is to use minItems (whose default is 0), such that an item will only be required if it has an index less than minItems (so to get required array items, you now need minItems).

[n3ps]

Looks good, thanks again! I'll push a minor update.