[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.

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