[nullsafe] x-plat friendly JSON data for nullsafe metainfo

Summary:
As artempyanykh pointed out, `nullable` works much better for required fields
than `optional` in terms of x-plat.

Reviewed By: artempyanykh

Differential Revision: D21129614

fbshipit-source-id: b03c91b78

infer/src/atd/jsonbug.atd

@@ -25,12 +25,12 @@ type nullsafe_mode = [Default | LocalTrustAll | LocalTrustSome | LocalTrustNone
 type nullsafe_meta_issue_info = {
   num_issues: int;
   curr_nullsafe_mode: nullsafe_mode;
-  can_be_promoted_to: nullsafe_mode option (* If present, it implies that the class is not only free of issues, but the mode can be promoted to a stricter one *)
+  can_be_promoted_to: nullsafe_mode nullable (* If present, it implies that the class is not only free of issues, but the mode can be promoted to a stricter one *)
 }
 
 type nullsafe_extra = {
   class_name: string;
-  package: string option;
+  package: string nullable;
   ?meta_issue_info: nullsafe_meta_issue_info option (* Should be present if the issue is "nullsafe meta issue" *)
 }