Merge branch 'generate-valid-json' into 'master'

Generate valid json

This patch helps to be compatible to other programing languages as it improves the validation of hook data. It seems only ruby can handle 'nil' as value while other json decode function will fatal.

See merge request !182

CHANGELOG

@@ -49,6 +49,7 @@ v 7.9.0 (unreleased)
   - Add database migration to clean group duplicates with same path and name (Make sure you have a backup before update)
   - Starred projects page at dashboard
   - Make email display name configurable
+  - 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