Gets the state this annotation sets on the one it replies to (State, PDF 1.5), such as Accepted; when absent, the
default of its StateModel (Unmarked for Marked, None for Review), else null.
public string? State { get; }
ISO 32000-2 §12.5.6.4, Table 175 ("Default: Unmarked if StateModel is Marked; None if StateModel is Review"), and §12.5.6.3,
Table 174. Without a StateModel, which is then required, StateModelMissing is recorded. A state model the
specification does not define has no default.