In Spark we allow UDFs to declare its expected input types in order to apply type coercion. The expected input type parameter takes a Seq[DataType] and uses Nil when no type coercion is applied. It makes more sense to take Option[Seq[DataType]] instead, so we can differentiate a no-arg function vs function with no expected input type specified.

Author: Reynold Xin <rxin@databricks.com>

Closes #10504 from rxin/SPARK-12549.