fix NullType MatchError in JavaSchemaRDD when sql has null

Author: YanTangZhai

sql/core/src/main/scala/org/apache/spark/sql/package.scala

@@ -263,6 +263,16 @@ package object sql {
   @DeveloperApi
   val ShortType = catalyst.types.ShortType
 
+  /**
+   * :: DeveloperApi ::
+   *
+   * The data type representing `NULL` values.
+   *
+   * @group dataType
+   */
+  @DeveloperApi
+  val NullType = catalyst.types.NullType
+  
   /**
    * :: DeveloperApi ::
    *

sql/core/src/main/scala/org/apache/spark/sql/types/util/DataTypeConversions.scala

@@ -62,6 +62,7 @@ protected[sql] object DataTypeConversions {
     case IntegerType => JDataType.IntegerType
     case LongType => JDataType.LongType
     case ShortType => JDataType.ShortType
+    case NullType => JDataType.StringType
 
     case arrayType: ArrayType => JDataType.createArrayType(
         asJavaDataType(arrayType.elementType), arrayType.containsNull)