Gets the document's optional content (layers), or null when the catalog has no OCProperties.
public PdfOptionalContentProperties? OptionalContent { get; }
ISO 32000-2 §8.11 and §7.7.2, Table 29 (PDF 1.5). Without OCProperties every optional content structure is ignored and all
content is visible (§8.11.4.2). The same view is returned while the catalog's OCProperties is the same dictionary and
nothing its group list was read from has changed; otherwise a new view is created.