[SPARK-19543] from_json fails when the input row is empty

## What changes were proposed in this pull request?

Using from_json on a column with an empty string results in: java.util.NoSuchElementException: head of empty list.

This is because `parser.parse(input)` may return `Nil` when `input.trim.isEmpty`

## How was this patch tested?

Regression test in `JsonExpressionsSuite`

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #16881 from brkyvz/json-fix.

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

@@ -496,7 +496,7 @@ case class JsonToStruct(schema: StructType, options: Map[String, String], child:
   override def dataType: DataType = schema
 
   override def nullSafeEval(json: Any): Any = {
-    try parser.parse(json.toString).head catch {
+    try parser.parse(json.toString).headOption.orNull catch {
       case _: SparkSQLJsonProcessingException => null
     }
   }

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/expressions/JsonExpressionsSuite.scala

@@ -376,6 +376,14 @@ class JsonExpressionsSuite extends SparkFunSuite with ExpressionEvalHelper {
     )
   }
 
+  test("SPARK-19543: from_json empty input column") {
+    val schema = StructType(StructField("a", IntegerType) :: Nil)
+    checkEvaluation(
+      JsonToStruct(schema, Map.empty, Literal.create(" ", StringType)),
+      null
+    )
+  }
+
   test("to_json") {
     val schema = StructType(StructField("a", IntegerType) :: Nil)
     val struct = Literal.create(create_row(1), schema)