In my interpretation of the specification, a PDF *might* actually contain a valid AcroForm dictionary even when `fields.length === 0`.
One example of this is the file: http://itext-general.2136553.n4.nabble.com/attachment/2172109/0/FormFile.pdf (from http://itext-general.2136553.n4.nabble.com/AcroForm-with-empty-Fields-array-td2172109.html).

With the above line, this will no longer trigger a warning. Is this intended?