Use `default_value_for` to handle null values in only_allow_merge_if_all_discussions_are_resolved

app/models/project.rb

@@ -35,6 +35,7 @@ class Project < ActiveRecord::Base
   default_value_for :builds_enabled, gitlab_config_features.builds
   default_value_for :wiki_enabled, gitlab_config_features.wiki
   default_value_for :snippets_enabled, gitlab_config_features.snippets
+  default_value_for :only_allow_merge_if_all_discussions_are_resolved, false
   after_create :ensure_dir_exist
   after_create :create_project_feature, unless: :project_feature
@@ -1334,10 +1335,6 @@ class Project < ActiveRecord::Base
     end
   end
-  def only_allow_merge_if_all_discussions_are_resolved
-    super || false
-  end
-
   private
   def pushes_since_gc_redis_key