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

## What changes were proposed in this pull request?
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.

## How was this patch tested?

unit tests

Closes apache#23164 from wangyum/SPARK-26198.

Authored-by: Yuming Wang <yumwang@ebay.com>
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 =>

sql/catalyst/src/test/scala/org/apache/spark/sql/types/MetadataSuite.scala

@@ -26,6 +26,7 @@ class MetadataSuite extends SparkFunSuite {
     assert(meta.## !== 0)
     assert(meta.getString("key") === "value")
     assert(meta.contains("key"))
+    assert(meta === Metadata.fromJson(meta.json))
     intercept[NoSuchElementException](meta.getString("no_such_key"))
     intercept[ClassCastException](meta.getBoolean("key"))
   }
@@ -36,6 +37,7 @@ class MetadataSuite extends SparkFunSuite {
     assert(meta.## !== 0)
     assert(meta.getLong("key") === 12)
     assert(meta.contains("key"))
+    assert(meta === Metadata.fromJson(meta.json))
     intercept[NoSuchElementException](meta.getLong("no_such_key"))
     intercept[ClassCastException](meta.getBoolean("key"))
   }
@@ -46,6 +48,7 @@ class MetadataSuite extends SparkFunSuite {
     assert(meta.## !== 0)
     assert(meta.getDouble("key") === 12)
     assert(meta.contains("key"))
+    assert(meta === Metadata.fromJson(meta.json))
     intercept[NoSuchElementException](meta.getDouble("no_such_key"))
     intercept[ClassCastException](meta.getBoolean("key"))
   }
@@ -56,6 +59,7 @@ class MetadataSuite extends SparkFunSuite {
     assert(meta.## !== 0)
     assert(meta.getBoolean("key") === true)
     assert(meta.contains("key"))
+    assert(meta === Metadata.fromJson(meta.json))
     intercept[NoSuchElementException](meta.getBoolean("no_such_key"))
     intercept[ClassCastException](meta.getString("key"))
   }
@@ -69,6 +73,7 @@ class MetadataSuite extends SparkFunSuite {
     assert(meta.getLong("key") === 0)
     assert(meta.getBoolean("key") === false)
     assert(meta.contains("key"))
+    assert(meta === Metadata.fromJson(meta.json))
     intercept[NoSuchElementException](meta.getLong("no_such_key"))
   }
 }