[nullsafe] Conventional style output of the field json

Summary:
Otherwise this gets serialized in an unconventional way; `nullable` serializes it as a
standard `null` or string value.

Reviewed By: artempyanykh

Differential Revision: D24827593

fbshipit-source-id: a5a9afc80

infer/src/atd/jsonbug.atd

@@ -58,7 +58,7 @@ type annotation_point = {
 
 type field_name = {
   class_name: string;
-  package_name: string option;
+  package_name: string nullable;
   field: string;
 }