[SPARK-21677][SQL] json_tuple throws NullPointException when column is null as string type

## What changes were proposed in this pull request?
``` scala
scala> Seq(("""{"Hyukjin": 224, "John": 1225}""")).toDS.selectExpr("json_tuple(value, trim(null))").show()
...
java.lang.NullPointerException
	at ...
```

Currently the `null` field name will throw NullPointException. As a given field name null can't be matched with any field names in json, we just output null as its column value. This PR achieves it by returning a very unlikely column name `__NullFieldName` in evaluation of the field names.

## How was this patch tested?
Added unit test.

Author: Jen-Ming Chung <jenmingisme@gmail.com>

Closes #18930 from jmchung/SPARK-21677.