[SPARK-8879][SQL] Remove EmptyRow class.

As a baby step towards no megamorphic InternalRow.

Author: Reynold Xin <rxin@databricks.com>

Closes #7277 from rxin/remove-empty-row and squashes the following commits:

594100e [Reynold Xin] [SPARK-8879][SQL] Remove EmptyRow class.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/complexTypeCreator.scala

@@ -120,4 +120,6 @@ case class CreateNamedStruct(children: Seq[Expression]) extends Expression {
   override def eval(input: InternalRow): Any = {
     InternalRow(valExprs.map(_.eval(input)): _*)
   }
+
+  override def prettyName: String = "named_struct"
 }

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/package.scala

@@ -17,6 +17,8 @@
 
 package org.apache.spark.sql.catalyst
 
+import org.apache.spark.sql.catalyst.expressions._
+
 /**
  * A set of classes that can be used to represent trees of relational expressions.  A key goal of
  * the expression library is to hide the details of naming and scoping from developers who want to
@@ -49,6 +51,11 @@ package org.apache.spark.sql.catalyst
  */
 package object expressions  {
 
+  /**
+   * Used as input into expressions whose output does not depend on any input value.
+   */
+  val EmptyRow: InternalRow = null
+
   /**
    * Converts a [[InternalRow]] to another Row given a sequence of expression that define each
    * column of the new row. If the schema of the input row is specified, then the given expression

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/rows.scala

@@ -54,26 +54,6 @@ abstract class MutableRow extends InternalRow {
   }
 }
 
-/**
- * A row with no data.  Calling any methods will result in an error.  Can be used as a placeholder.
- */
-object EmptyRow extends InternalRow {
-  override def apply(i: Int): Any = throw new UnsupportedOperationException
-  override def toSeq: Seq[Any] = Seq.empty
-  override def length: Int = 0
-  override def isNullAt(i: Int): Boolean = throw new UnsupportedOperationException
-  override def getInt(i: Int): Int = throw new UnsupportedOperationException
-  override def getLong(i: Int): Long = throw new UnsupportedOperationException
-  override def getDouble(i: Int): Double = throw new UnsupportedOperationException
-  override def getFloat(i: Int): Float = throw new UnsupportedOperationException
-  override def getBoolean(i: Int): Boolean = throw new UnsupportedOperationException
-  override def getShort(i: Int): Short = throw new UnsupportedOperationException
-  override def getByte(i: Int): Byte = throw new UnsupportedOperationException
-  override def getString(i: Int): String = throw new UnsupportedOperationException
-  override def getAs[T](i: Int): T = throw new UnsupportedOperationException
-  override def copy(): InternalRow = this
-}
-
 /**
  * A row implementation that uses an array of objects as the underlying storage.
  */