[SPARK-26198][SQL] Fix Metadata serialize null values throw NPE

How to reproduce this issue:
```scala
scala> val meta = new org.apache.spark.sql.types.MetadataBuilder().putNull("key").build().json
java.lang.NullPointerException
  at org.apache.spark.sql.types.Metadata$.org$apache$spark$sql$types$Metadata$$toJsonValue(Metadata.scala:196)
  at org.apache.spark.sql.types.Metadata$$anonfun$1.apply(Metadata.scala:180)
```

This pr fix `NullPointerException` when `Metadata` serialize `null` values.

unit tests

Closes #23164 from wangyum/SPARK-26198.

Authored-by: Yuming Wang <yumwang@ebay.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>
(cherry picked from commit 676bbb2)
Signed-off-by: Sean Owen <sean.owen@databricks.com>

sql/catalyst/src/main/scala/org/apache/spark/sql/types/Metadata.scala

@@ -190,6 +190,8 @@ object Metadata {
         JBool(x)
       case x: String =>
         JString(x)
+      case null =>
+        JNull
       case x: Metadata =>
         toJsonValue(x.map)
       case other =>