Fix json_tuple throw NPE if fields exist no foldable null value

SELECT json_tuple('{"a":"1"}', if(c1 < 1, null, 'a')) FROM ( SELECT rand() AS c1 );

Caused by: java.lang.NullPointerException
	at org.apache.spark.sql.catalyst.expressions.JsonTuple.$anonfun$parseRow$2(jsonExpressions.scala:435)
	at scala.collection.TraversableLike.$anonfun$map$1(TraversableLike.scala:286)
	at scala.collection.mutable.ResizableArray.foreach(ResizableArray.scala:62)
	at scala.collection.mutable.ResizableArray.foreach$(ResizableArray.scala:55)
	at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:49)
	at scala.collection.TraversableLike.map(TraversableLike.scala:286)
	at scala.collection.TraversableLike.map$(TraversableLike.scala:279)
	at scala.collection.AbstractTraversable.map(Traversable.scala:108)
	at org.apache.spark.sql.catalyst.expressions.JsonTuple.parseRow(jsonExpressions.scala:435)
	at org.apache.spark.sql.catalyst.expressions.JsonTuple.$anonfun$eval$6(jsonExpressions.scala:413)