[SPARK-18583][SQL] Fix nullability of InputFileName.

## What changes were proposed in this pull request?

The nullability of `InputFileName` should be `false`.

## How was this patch tested?

Existing tests.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #16007 from ueshin/issues/SPARK-18583.

(cherry picked from commit a88329d4)
Signed-off-by: Reynold Xin <rxin@databricks.com>

core/src/main/scala/org/apache/spark/rdd/InputFileNameHolder.scala

@@ -22,6 +22,8 @@ import org.apache.spark.unsafe.types.UTF8String
 /**
  * This holds file names of the current Spark task. This is used in HadoopRDD,
  * FileScanRDD, NewHadoopRDD and InputFileName function in Spark SQL.
+ *
+ * The returned value should never be null but empty string if it is unknown.
  */
 private[spark] object InputFileNameHolder {
   /**
@@ -32,9 +34,15 @@ private[spark] object InputFileNameHolder {
     override protected def initialValue(): UTF8String = UTF8String.fromString("")
   }
 
+  /**
+   * Returns the holding file name or empty string if it is unknown.
+   */
   def getInputFileName(): UTF8String = inputFileName.get()
 
-  private[spark] def setInputFileName(file: String) = inputFileName.set(UTF8String.fromString(file))
+  private[spark] def setInputFileName(file: String) = {
+    require(file != null, "The input file name cannot be null")
+    inputFileName.set(UTF8String.fromString(file))
+  }
 
   private[spark] def unsetInputFileName(): Unit = inputFileName.remove()
 

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

@@ -30,7 +30,7 @@ import org.apache.spark.unsafe.types.UTF8String
   usage = "_FUNC_() - Returns the name of the current file being read if available.")
 case class InputFileName() extends LeafExpression with Nondeterministic {
 
-  override def nullable: Boolean = true
+  override def nullable: Boolean = false
 
   override def dataType: DataType = StringType