Converts this to a Codec[Unit]
that encodes using the zero value of the implicitly
available Monoid[A]
and decodes a unit value when this codec decodes an A
successfully.
(codecSyntax: TransformSyntax[CodecSyntax, A]).self
(Since version 1.7.0) Use widenOpt instead
Extension methods for a
Codec[A]
.