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

[gatorsmile]

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

    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()

    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.

[SparkQA]

Test build #46827 has finished for PR 10018 at commit 54edc84.

• This patch fails from timeout after a configured wait of 250m.
• This patch merges cleanly.
• This patch adds no public classes.

[gatorsmile]

retest this please

[SparkQA]

Test build #46832 has finished for PR 10018 at commit 54edc84.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[gatorsmile]

@rxin @davies Could you help me review the code changes for fixing get_json_object? The bug was discovered in the PR: #9977.

Thank you very much!

[davies]

LGTM