Avoid orNull in TypedConfigBuilder and OptionalConfigEntry

diff --git a/core/src/main/scala/org/apache/spark/internal/config/ConfigBuilder.scala b/core/src/main/scala/org/apache/spark/internal/config/ConfigBuilder.scala
index 1f19e9444d38..d06535722625 100644
--- a/core/src/main/scala/org/apache/spark/internal/config/ConfigBuilder.scala
+++ b/core/src/main/scala/org/apache/spark/internal/config/ConfigBuilder.scala
@@ -94,7 +94,7 @@ private[spark] class TypedConfigBuilder[T](
   import ConfigHelpers._

   def this(parent: ConfigBuilder, converter: String => T) = {
-    this(parent, converter, Option(_).map(_.toString).orNull)
+    this(parent, converter, { v: T => v.toString })
   }

   /** Apply a transformation to the user-provided values of the config entry. */