*<Null safety>*
void encode (Encoder encoder, T? value, int offset) override
@override void encode(Encoder encoder, T? value, int offset) { if (value == null) { encoder.encodeUint64(kAllocAbsent, offset); } else { encoder.encodeUint64(kAllocPresent, offset); int childOffset = encoder.alloc(element.encodingInlineSize()); element.encode(encoder, value, childOffset); } }