Added missing test models for elasticsearch

tests/unit/data/ar/elasticsearch/OrderItemWithNullFK.php

+<?php
+
+namespace yiiunit\data\ar\elasticsearch;
+
+/**
+ * Class OrderItem
+ *
+ * @property integer $order_id
+ * @property integer $item_id
+ * @property integer $quantity
+ * @property string $subtotal
+ */
+class OrderItemWithNullFK extends ActiveRecord
+{
+    public function attributes()
+    {
+        return ['order_id', 'item_id', 'quantity', 'subtotal'];
+    }
+
+    public static function tableName()
+    {
+        return 'order_item_with_null_fk';
+    }
+}

tests/unit/data/ar/elasticsearch/OrderWithNullFK.php

+<?php
+
+namespace yiiunit\data\ar\elasticsearch;
+
+/**
+ * Class Order
+ *
+ * @property integer $id
+ * @property integer $customer_id
+ * @property integer $created_at
+ * @property string $total
+ */
+class OrderWithNullFK extends ActiveRecord
+{
+    public static function primaryKey()
+    {
+        return ['id'];
+    }
+
+    public function attributes()
+    {
+        return ['id', 'customer_id', 'created_at', 'total'];
+    }
+
+    public static function tableName()
+    {
+        return 'order_with_null_fk';
+    }
+}