Better handling of json schema of type array

[ingo-eichhorst]

If the method "fromJsonSchema" is called with a json schema of type array the method returns an empty object. After I had figured this out not supporting json schemas of type array definitely makes sense to me. But it could save the next developers some time if there is an error message instead.

Steps to reporoduce:

• instead of a json schema with root level: "type": "object"
• use a json schema with: "type": "array"