Modify test to pass new project hooks

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/observers/merge_request_observer.rb

@@ -64,6 +64,8 @@ class MergeRequestObserver < ActivityObserver
   end
 
   def execute_hooks(merge_request)
+    if merge_request.project
       merge_request.project.execute_hooks(merge_request.to_hook_data, :merge_request_hooks)
     end
+  end
 end

spec/observers/merge_request_observer_spec.rb

@@ -4,7 +4,7 @@ describe MergeRequestObserver do
   let(:some_user) { create :user }
   let(:assignee) { create :user }
   let(:author) { create :user }
-  let(:mr_mock) { double(:merge_request, id: 42, assignee: assignee, author: author) }
+  let(:mr_mock) { double(:merge_request, id: 42, assignee: assignee, author: author).as_null_object }
   let(:assigned_mr) { create(:merge_request, assignee: assignee, author: author, target_project: create(:project)) }
   let(:unassigned_mr) { create(:merge_request, author: author, target_project: create(:project)) }
   let(:closed_assigned_mr) { create(:closed_merge_request, assignee: assignee, author: author, target_project: create(:project)) }