Generate valid json for hooks

It seems that ruby can handle 'nil' value but other json processors
(like PHP) throw an error. This is always generated for empty arrays.

CHANGELOG

@@ -35,6 +35,7 @@ v 7.9.0 (unreleased)
   - Move groups page from profile to dashboard
   - Starred projects page at dashboard 
   - Blocking user does not remove him/her from project/groups but show blocked label
+  - Improve json validation in hook data
 
 v 7.8.2
   - Fix service migration issue when upgrading from versions prior to 7.3

app/services/system_hooks_service.rb

@@ -41,7 +41,7 @@ class SystemHooksService
         path_with_namespace: model.path_with_namespace,
         project_id: model.id,
         owner_name: owner.name,
-        owner_email: owner.respond_to?(:email) ?  owner.email : nil,
+        owner_email: owner.respond_to?(:email) ?  owner.email : "",
         project_visibility: Project.visibility_levels.key(model.visibility_level_field).downcase
       })
     when User

lib/gitlab/push_data_builder.rb

@@ -58,6 +58,7 @@ module Gitlab
           data[:commits] << commit.hook_attrs(project)
         end
 
+        data[:commits] = "" if data[:commits].count == 0
         data
       end