clarify that type must be a non-empty array

[gregsdennis]

What kind of change does this PR introduce?

clarification

Issue & Discussion References

• Closes Spec and meta-schema seem inconsistent w.r.t empty "type" array #1404

Summary

Clarifies that type as an array must be non-empty.

Does this PR introduce a breaking change?

Technically yes? @awwright did point out that there is a niche use case as an alternative to {not:{}}, but the keyword certainly wasn't intended to be used this way. Also, given that we present {not:{}} as the analog to false, it's extremely unlikely that anyone will be using it this way on purpose.

[jviotti]

This is great. All of these edge cases and what is technically possible can be very confusing.

[jdesrosiers]

I'm a little reluctant to approve a breaking change without documented consensus and an ADR, but I think it's ok in this case because the support for type with an empty array was never reliable because the meta-schema has included "minItems": 1 since at least draft-04 (I didn't bother looking further back). That means that implementations that use the meta-schema to validate schemas (probably most of them) have been rejecting type with an empty array.