[SPARK-27971][SQL][R] MapPartitionsInRWithArrowExec.evaluate shouldn't eagerly read the first batch

## What changes were proposed in this pull request?

This PR is the same fix as https://github.com/apache/spark/pull/24816 but in vectorized `dapply` in SparkR.

## How was this patch tested?

Manually tested.

Closes #24818 from HyukjinKwon/SPARK-27971.

Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

sql/core/src/main/scala/org/apache/spark/sql/execution/objects.scala

@@ -243,28 +243,11 @@ case class MapPartitionsInRWithArrowExec(
       // binary in a batch due to the limitation of R API. See also ARROW-4512.
       val columnarBatchIter = runner.compute(batchIter, -1)
       val outputProject = UnsafeProjection.create(output, output)
-      new Iterator[InternalRow] {
-
-        private var currentIter = if (columnarBatchIter.hasNext) {
-          val batch = columnarBatchIter.next()
-          val actualDataTypes = (0 until batch.numCols()).map(i => batch.column(i).dataType())
-          assert(outputTypes == actualDataTypes, "Invalid schema from dapply(): " +
-            s"expected ${outputTypes.mkString(", ")}, got ${actualDataTypes.mkString(", ")}")
-          batch.rowIterator.asScala
-        } else {
-          Iterator.empty
-        }
-
-        override def hasNext: Boolean = currentIter.hasNext || {
-          if (columnarBatchIter.hasNext) {
-            currentIter = columnarBatchIter.next().rowIterator.asScala
-            hasNext
-          } else {
-            false
-          }
-        }
-
-        override def next(): InternalRow = currentIter.next()
+      columnarBatchIter.flatMap { batch =>
+        val actualDataTypes = (0 until batch.numCols()).map(i => batch.column(i).dataType())
+        assert(outputTypes == actualDataTypes, "Invalid schema from dapply(): " +
+          s"expected ${outputTypes.mkString(", ")}, got ${actualDataTypes.mkString(", ")}")
+        batch.rowIterator.asScala
       }.map(outputProject)
     }
   }