Remove null from enum class

Summary:
facebook/facebook-java-business-sdk#235

Looks like we don't need `NULL` for enum class

Reviewed By: joesus

Differential Revision: D17242244

fbshipit-source-id: 33c2cc4c

Author: jingping2015

templates/java/src/main/java/com/facebook/ads/sdk/codegen.mustache

@@ -626,7 +626,7 @@ public class {{name:pascal_case}} extends APINode {
       @SerializedName("{{{value}}}")
       VALUE_{{{value:upper_case}}}("{{{value}}}"),
       {{/values_with_naming_convention}}
-      NULL(null);
+      ;
 
       private String value;