Support for strict OOXML

There are 2 types of OOXML format: transitional and strict. Transitional is what's mostly used but some softwares still allow XLSX to be generated using the strict OOXML format.
In this format, namespaces of the XML files are different: `http://schemas.openxmlformats.org/officeDocument/2006/relationships/sharedStrings` is replaced by `http://purl.oclc.org/ooxml/officeDocument/relationships/sharedStrings` for instance. To support both formats, Spout needs to be able to look for both.

src/Spout/Common/Entity/Cell.php

@@ -65,7 +65,7 @@ class Cell
     protected $style;
 
     /**
-     * @param $value mixed
+     * @param mixed|null $value
      * @param Style|null $style
      */
     public function __construct($value, Style $style = null)