nullable method
Handles SQL NULL before delegating non-null values to this codec.
Nullable wrappers share a comparison identity with each other, distinct from a decoder that handles SQL NULL itself.
Implementation
Codec<T?> nullable() => Codec._nullable(
sqlType,
(value) => value == null ? null : decode(value),
(value) => value == null ? null : encode(value),
_identity ?? (this, #nullable),
);