Fix eager loading bug: associations with primary key of 0 were not inclunded

lib/dao.js

@@ -249,7 +249,7 @@ module.exports = (function() {
           includeValidated: true,
           raw: options.raw
         })
-        , isEmpty = !Utils.firstValueOfHash(daoInstance.identifiers)
+        , isEmpty = Utils.firstValueOfHash(daoInstance.identifiers) === null
 
       if (association.isSingleAssociation) {
         accessor = Utils.singularize(accessor, self.sequelize.language)