Gets the glyph id in Program: 0, the missing glyph (.notdef), when the code selects none.
.notdef
public int GlyphId { get; }
ISO 32000-2 §9.6.5 (simple fonts), §9.7.4.2 (CIDFonts) and §9.7.6.3 (the CID 0 glyph for an unmapped code).