Unselect multiselect on product edit view

Fix for issue magento#8577 : magento#8577

Preconditions
Install Magento 2
Create a product with a multiselect attribute AND selected values.
Save It

Steps to reproduce
Unselect ALL values
Save product again

Expected result
All values should be unselected.

Actual result
Previous values are still selected.

Author: syno-dlr

app/code/Magento/Catalog/Controller/Adminhtml/Product/Initialization/Helper.php

@@ -142,6 +142,11 @@ public function initializeFromData(\Magento\Catalog\Model\Product $product, arra
         $dateFieldFilters = [];
         $attributes = $product->getAttributes();
         foreach ($attributes as $attrKey => $attribute) {
+            if ($attribute->getFrontendInput() == 'multiselect') {
+                if (empty($productData[$attrKey])) {
+                    $productData[$attrKey] = '';
+                }
+            }
             if ($attribute->getBackend()->getType() == 'datetime') {
                 if (array_key_exists($attrKey, $productData) && $productData[$attrKey] != '') {
                     $dateFieldFilters[$attrKey] = $this->getDateTimeFilter();