Gets the name of the stamp's icon (Name), such as Approved or Draft (the default); null when
the intent is present and not Stamp, such as StampImage or StampSnapshot, where a Name entry is ignored with a diagnostic.
public string? IconName { get; }
ISO 32000-2 §12.5.6.12, Table 184: Name shall not be present when IT is present and not Stamp (PDF 2.0).