[SPARK-41959][SQL] Improve v1 writes with empty2null

### What changes were proposed in this pull request?

Cleanup some unnecessary `Empty2Null` related code

### Why are the changes needed?

V1Writes checked idempotency using WriteFiles, so it's unnecessary to check if empty2null if exists again.

### Does this PR introduce _any_ user-facing change?

no

### How was this patch tested?

pass CI

Closes #39475 from ulysses-you/SPARK-41959.

Authored-by: ulysses-you <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: ulysses-you

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/FileFormatWriter.scala

@@ -206,13 +206,8 @@ object FileFormatWriter extends Logging {
       partitionColumns: Seq[Attribute],
       sortColumns: Seq[Attribute],
       orderingMatched: Boolean): Set[String] = {
-    val hasEmpty2Null = plan.exists(p => V1WritesUtils.hasEmptyToNull(p.expressions))
-    val empty2NullPlan = if (hasEmpty2Null) {
-      plan
-    } else {
-      val projectList = V1WritesUtils.convertEmptyToNull(plan.output, partitionColumns)
-      if (projectList.nonEmpty) ProjectExec(projectList, plan) else plan
-    }
+    val projectList = V1WritesUtils.convertEmptyToNull(plan.output, partitionColumns)
+    val empty2NullPlan = if (projectList.nonEmpty) ProjectExec(projectList, plan) else plan
 
     writeAndCommit(job, description, committer) {
       val (planToExecute, concurrentOutputWriterSpec) = if (orderingMatched) {

sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/V1Writes.scala

@@ -93,13 +93,8 @@ object V1Writes extends Rule[LogicalPlan] with SQLConfHelper {
   }
 
   private def prepareQuery(write: V1WriteCommand, query: LogicalPlan): LogicalPlan = {
-    val hasEmpty2Null = query.exists(p => hasEmptyToNull(p.expressions))
-    val empty2NullPlan = if (hasEmpty2Null) {
-      query
-    } else {
-      val projectList = convertEmptyToNull(query.output, write.partitionColumns)
-      if (projectList.isEmpty) query else Project(projectList, query)
-    }
+    val projectList = convertEmptyToNull(query.output, write.partitionColumns)
+    val empty2NullPlan = if (projectList.isEmpty) query else Project(projectList, query)
     assert(empty2NullPlan.output.length == query.output.length)
     val attrMap = AttributeMap(query.output.zip(empty2NullPlan.output))
 
@@ -108,7 +103,6 @@ object V1Writes extends Rule[LogicalPlan] with SQLConfHelper {
       case a: Attribute => attrMap.getOrElse(a, a)
     }.asInstanceOf[SortOrder])
     val outputOrdering = query.outputOrdering
-    // Check if the ordering is already matched to ensure the idempotency of the rule.
     val orderingMatched = isOrderingMatched(requiredOrdering, outputOrdering)
     if (orderingMatched) {
       empty2NullPlan