[SPARK-25044][SQL][FOLLOWUP] add back UserDefinedFunction.inputTypes

## What changes were proposed in this pull request?

This is a followup of #22259 .

Scala case class has a wide surface: apply, unapply, accessors, copy, etc.

In #22259 , we change the type of `UserDefinedFunction.inputTypes` from `Option[Seq[DataType]]` to `Option[Seq[Schema]]`. This breaks backward compatibility.

This PR changes the type back, and use a `var` to keep the new nullable info.

## How was this patch tested?

N/A

Closes #22319 from cloud-fan/revert.

Authored-by: Wenchen Fan <wenchen@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

Author: cloud-fan

project/MimaExcludes.scala

@@ -47,11 +47,6 @@ object MimaExcludes {
     ProblemFilters.exclude[DirectMissingMethodProblem]("org.apache.spark.ml.fpm.FPGrowthModel.this"),
     ProblemFilters.exclude[DirectMissingMethodProblem]("org.apache.spark.mllib.fpm.AssociationRules#Rule.this"),
 
-    // [SPARK-25044] Address translation of LMF closure primitive args to Object in Scala 2.12
-    ProblemFilters.exclude[MissingTypesProblem]("org.apache.spark.sql.expressions.UserDefinedFunction$"),
-    ProblemFilters.exclude[DirectMissingMethodProblem]("org.apache.spark.sql.expressions.UserDefinedFunction.apply"),
-    ProblemFilters.exclude[DirectMissingMethodProblem]("org.apache.spark.sql.expressions.UserDefinedFunction.copy"),
-
     // [SPARK-24296][CORE] Replicate large blocks as a stream.
     ProblemFilters.exclude[DirectMissingMethodProblem]("org.apache.spark.network.netty.NettyBlockRpcServer.this"),
     // [SPARK-23528] Add numIter to ClusteringSummary