Should this be an empty array ?

An empty array would work. The problem solved here is when a field defined as group or nested is null in a document, like onepassword.details here elastic/integrations@09a01f2#diff-76a51622c000706ccf925266e2e1255f954f7b55380135342a9d51aa3b81b5cdL55

Ok, I misunderstood the definition field. I thought it was resulting definition built.

Just to be sure, it's being compared the value nil of a field null_array against that definition/schema in definition. Would that be right?

Yes, this would be like validating a document with this JSON content:

{
  "null_array": null
}

With this fields definition:

- name: null_array
  type: group
  fields: ...

Validation fails without this change.

Thanks!