EQL v3 empty JSON arrays break array accessors

[tobyhede]

The EQL v3 JSON domain accepts an empty searchable vector, but ste_vec()
aggregates zero rows to SQL NULL. As a result, jsonb_array_length returns
NULL instead of zero and jsonb_array_elements raises SQLSTATE 22004 because
its FOR loop has a null upper bound.

Acceptance criteria

• The length of an encrypted empty JSON array is zero.
• jsonb_array_elements returns zero rows without error.
• Populated-array behaviour is unchanged.
• Tests exercise a real encrypted empty-array payload as well as the SQL helper
  boundary.
• The implementation composes with the umbrella array aggregation design.