[SPARK-25044][FOLLOW-UP] Change ScalaUDF constructor signature
## What changes were proposed in this pull request?
This is a follow-up PR for #22259. The extra field added in `ScalaUDF` with the
original PR was declared optional, but should be indeed required, otherwise
callers of `ScalaUDF`'s
[SPARK-25044][FOLLOW-UP] Change ScalaUDF constructor signature
## What changes were proposed in this pull request?
This is a follow-up PR for #22259. The extra field added in `ScalaUDF` with the
original PR was declared optional, but should be indeed required, otherwise
callers of `ScalaUDF`'s
