Gets the document's interactive form, or null when the catalog has no AcroForm dictionary.
public PdfAcroForm? AcroForm { get; }
ISO 32000-2 §7.7.2, Table 29 (PDF 1.2), and §12.7.3. The same view is returned while the catalog's AcroForm is the same
dictionary. An entry that is not a dictionary reads as absent with an AcroFormInvalid diagnostic.