Gets the file identifier (the trailer's ID), or null when the trailer has none or it is not two strings.
public PdfFileIdentifier? FileIdentifier { get; }
ISO 32000-2 §14.4 and §7.5.5, Table 15: an array of two byte strings, each at least 16 bytes. Shorter strings are kept as they
are; an entry that is not two strings is ignored. Both are reported as FileIdentifierInvalid.