perf(query): optimize groupJoinData to use a single loop

lib/instance.js

@@ -361,7 +361,7 @@ module.exports = (function() {
       , accessor             = Utils._.camelize(key)
       , childOptions
       , primaryKeyAttribute  = include.model.primaryKeyAttribute
-      , isEmpty              = value[0] && value[0][primaryKeyAttribute] === null
+      , isEmpty
 
     if (!isEmpty) {
       childOptions = {
@@ -380,9 +380,15 @@ module.exports = (function() {
       accessor = accessor.slice(0,1).toLowerCase() + accessor.slice(1)
 
       if (association.isSingleAssociation) {
+        if (Array.isArray(value)) {
+          value = value[0];
+        }
+
+        isEmpty = value && value[primaryKeyAttribute] === null
         accessor = Utils.singularize(accessor, self.Model.options.language)
-        self[accessor] = self.dataValues[accessor] = isEmpty ? null : include.model.build(value[0], childOptions)
+        self[accessor] = self.dataValues[accessor] = isEmpty ? null : include.model.build(value, childOptions)
       } else {
+        isEmpty = value[0] && value[0][primaryKeyAttribute] === null
         self[accessor] = self.dataValues[accessor] = isEmpty ? [] : include.model.bulkBuild(value, childOptions)
       }
     }