Fixed deprecation warning on Mage_Catalog_Helper_Image (#3201)

Author: fballiano

app/code/core/Mage/Catalog/Helper/Image.php

@@ -564,13 +564,18 @@ protected function getImageFile()
      */
     protected function parseSize($string)
     {
+        if ($string === null) {
+            return false;
+        }
+
         $size = explode('x', strtolower($string));
         if (count($size) === 2) {
             return [
                 'width' => ($size[0] > 0) ? $size[0] : null,
                 'heigth' => ($size[1] > 0) ? $size[1] : null,
             ];
         }
+
         return false;
     }