Gets the document's metadata stream (the catalog's Metadata), or null when the document has none.
public PdfMetadata? Metadata { get; }
ISO 32000-2 §14.3.2, Table 347. An entry that is not a stream is ignored with a MetadataStreamInvalid diagnostic; a
stream whose Type is not Metadata or whose Subtype is not XML is read with a
MetadataStreamTypeInvalid diagnostic.