rule: distinguish undeclared vs empty local filter

in the presence of a declared default_branch_filter,
empty ([]) = locally override it and apply no filter
undefined  = use default_branch_filter

Author: yasunariw

lib/rule.atd

@@ -66,7 +66,7 @@ type status_rule = {
 type prefix_rule = {
   ?allow <json name="match"> : string list nullable;
   ?ignore : string list nullable;
-  ~branch_filters <ocaml default="[]"> : string list;
+  ?branch_filters : string list nullable;
   channel_name <json name="channel"> : string;
 }
 

lib/rule.ml

@@ -44,8 +44,9 @@ module Prefix = struct
       if no filter is matched. *)
   let filter_by_branch ~branch ~default_branch_filters ~distinct rule =
     match rule.branch_filters with
-    | _ :: _ as filters -> List.mem filters branch ~equal:String.equal
-    | [] ->
+    | Some (_ :: _ as filters) -> List.mem filters branch ~equal:String.equal
+    | Some [] -> distinct
+    | None ->
     match default_branch_filters with
     | _ :: _ as filters -> List.mem filters branch ~equal:String.equal
     | [] -> distinct