Empty rows do not need to be written for XLSX files (#336)

src/Spout/Writer/Common/Helper/CellHelper.php

@@ -46,6 +46,15 @@ class CellHelper
         return self::$columnIndexToCellIndexCache[$originalColumnIndex];
     }
 
+    /**
+     * @param $value
+     * @return bool Whether the given value is considered "empty"
+     */
+    public static function isEmpty($value)
+    {
+        return ($value === null || $value === '');
+    }
+
     /**
      * @param $value
      * @return bool Whether the given value is a non empty string