in core/src/main/scala/com/gu/marley/AvroSerialisable.scala [67:71]
override def writableValue(t: Option[T]): Any = t.map(w.writableValue).orNull
override def read(x: Any): Option[T] = Option(x).map(w.read)
}
def SeqAvroSerialisable[T](w: AvroSerialisable[T]) = new AvroSerialisable[collection.Seq[T]] {