asn1: Add support for NULL

[facutuesca]

This adds support for encoding/decoding NULL.

here's an example of how it would be used:

@asn1.sequence
class MySeq:
    field: asn1.Null

object = MySeq(field=asn1.Null())
encoded = encode_der(object)

decoded = decode_der(MySeq, encoded)
assert decoded.field == asn1.Null()

Part of #12283