[SPARK-12028] [SQL] get_json_object returns an incorrect result when the value is null literals

When calling `get_json_object` for the following two cases, both results are `"null"`:

```scala
    val tuple: Seq[(String, String)] = ("5", """{"f1": null}""") :: Nil
    val df: DataFrame = tuple.toDF("key", "jstring")
    val res = df.select(functions.get_json_object($"jstring", "$.f1")).collect()
```
```scala
    val tuple2: Seq[(String, String)] = ("5", """{"f1": "null"}""") :: Nil
    val df2: DataFrame = tuple2.toDF("key", "jstring")
    val res3 = df2.select(functions.get_json_object($"jstring", "$.f1")).collect()
```

Fixed the problem and also added a test case.

Author: gatorsmile <gatorsmile@gmail.com>

Closes #10018 from gatorsmile/get_json_object.

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

@@ -298,8 +298,11 @@ case class GetJsonObject(json: Expression, path: Expression)
 
       case (FIELD_NAME, Named(name) :: xs) if p.getCurrentName == name =>
         // exact field match
-        p.nextToken()
-        evaluatePath(p, g, style, xs)
+        if (p.nextToken() != JsonToken.VALUE_NULL) {
+          evaluatePath(p, g, style, xs)
+        } else {
+          false
+        }
 
       case (FIELD_NAME, Wildcard :: xs) =>
         // wildcard field match

sql/core/src/test/scala/org/apache/spark/sql/JsonFunctionsSuite.scala

@@ -39,6 +39,26 @@ class JsonFunctionsSuite extends QueryTest with SharedSQLContext {
     ("6", "[invalid JSON string]") ::
     Nil
 
+  test("function get_json_object - null") {
+    val df: DataFrame = tuples.toDF("key", "jstring")
+    val expected =
+      Row("1", "value1", "value2", "3", null, "5.23") ::
+        Row("2", "value12", "2", "value3", "4.01", null) ::
+        Row("3", "value13", "2", "value33", "value44", "5.01") ::
+        Row("4", null, null, null, null, null) ::
+        Row("5", "", null, null, null, null) ::
+        Row("6", null, null, null, null, null) ::
+        Nil
+
+    checkAnswer(
+      df.select($"key", functions.get_json_object($"jstring", "$.f1"),
+        functions.get_json_object($"jstring", "$.f2"),
+        functions.get_json_object($"jstring", "$.f3"),
+        functions.get_json_object($"jstring", "$.f4"),
+        functions.get_json_object($"jstring", "$.f5")),
+      expected)
+  }
+
   test("json_tuple select") {
     val df: DataFrame = tuples.toDF("key", "jstring")
     val expected =