Gets the appearance dictionary (AP, PDF 1.2), or null when the annotation has none.
public PdfAppearanceDictionary? AppearanceDictionary { get; }
ISO 32000-2 §12.5.5, Table 170, and §12.5.2, Table 166. Every annotation shall have one except a popup, link or projection
annotation and one whose rectangle has zero width and zero height; that wording is PDF 2.0's, so a missing one records
AppearanceMissing only in a PDF 2.0 file.