## What changes were proposed in this pull request?
This PR proposes to match up the behaviour of `to_json` to `from_json` function for null-safety.
Currently, it throws `NullPointException` but this PR fixes this to produce `null` instead.
with the data below:
```scala
import spark.implicits._
val df = Seq(Some(Tuple1(Tuple1(1))), None).toDF("a")
df.show()
```
```
+----+
| a|
+----+
| [1]|
|null|
+----+
```
the codes below
```scala
import org.apache.spark.sql.functions._
df.select(to_json($"a")).show()
```
produces..
**Before**
throws `NullPointException` as below:
```
java.lang.NullPointerException
at org.apache.spark.sql.catalyst.json.JacksonGenerator.org$apache$spark$sql$catalyst$json$JacksonGenerator$$writeFields(JacksonGenerator.scala:138)
at org.apache.spark.sql.catalyst.json.JacksonGenerator$$anonfun$write$1.apply$mcV$sp(JacksonGenerator.scala:194)
at org.apache.spark.sql.catalyst.json.JacksonGenerator.org$apache$spark$sql$catalyst$json$JacksonGenerator$$writeObject(JacksonGenerator.scala:131)
at org.apache.spark.sql.catalyst.json.JacksonGenerator.write(JacksonGenerator.scala:193)
at org.apache.spark.sql.catalyst.expressions.StructToJson.eval(jsonExpressions.scala:544)
at org.apache.spark.sql.catalyst.expressions.Alias.eval(namedExpressions.scala:142)
at org.apache.spark.sql.catalyst.expressions.InterpretedProjection.apply(Projection.scala:48)
at org.apache.spark.sql.catalyst.expressions.InterpretedProjection.apply(Projection.scala:30)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
```
**After**
```
+---------------+
|structtojson(a)|
+---------------+
| {"_1":1}|
| null|
+---------------+
```
## How was this patch tested?
Unit test in `JsonExpressionsSuite.scala` and `JsonFunctionsSuite.scala`.
Author: hyukjinkwon <gurwls223@gmail.com>
Closes#15792 from HyukjinKwon/SPARK-18295.
3eda05703f