`required` array fields should be non-empty

[hughsimpson]

E.g. we should enforce that @type in

class FHIRSignature is Element is export {
          has Reference $.who is required;
          has Coding @.type is required;
          has DateTime $.when is required;
          has Base64Binary $.data;
          has FHIRCode $.sigFormat;
          has Reference $.onBehalfOf;
          has FHIRCode $.targetFormat;
}

should have at least one element