MAGETWO-64047: WebAPI: It's impossible to empty numeric fields after they're set

 - add handling of null values passed as 'special_price' custom attribute

Author: Cari Spruiell

app/code/Magento/Catalog/Observer/UnsetSpecialPrice.php

@@ -0,0 +1,31 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+namespace Magento\Catalog\Observer;
+
+use Magento\Framework\Event\ObserverInterface;
+
+/**
+ *  Unset value for Special Price if passed as null
+ */
+class UnsetSpecialPrice implements ObserverInterface
+{
+    /**
+     * Unset the Special Price attribute if it is null
+     *
+     * @param \Magento\Framework\Event\Observer $observer
+     * @return $this
+     */
+    public function execute(\Magento\Framework\Event\Observer $observer)
+    {
+        /** @var  $product \Magento\Catalog\Model\Product */
+        $product = $observer->getEvent()->getProduct();
+        if ($product->getSpecialPrice() === null) {
+            $product->setData('special_price', '');
+        }
+
+        return $this;
+    }
+}

app/code/Magento/Catalog/etc/events.xml

@@ -56,5 +56,6 @@
     </event>
     <event name="catalog_product_save_before">
         <observer name="set_special_price_start_date" instance="Magento\Catalog\Observer\SetSpecialPriceStartDate" />
+        <observer name="unset_special_price" instance="Magento\Catalog\Observer\UnsetSpecialPrice" />
     </event>
 </config>

app/code/Magento/Eav/Model/Entity/Attribute/AbstractAttribute.php

@@ -661,7 +661,7 @@ public function isValueEmpty($value)
      */
     public function isAllowedEmptyTextValue($value)
     {
-        return $this->isInEmptyStringTypes() && ($value === self::EMPTY_STRING || $value === null);
+        return $this->isInEmptyStringTypes() && $value === self::EMPTY_STRING;
     }
 
     /**