Set "$contentRange" default to empty string if header key not found. …

…Passing "null" to the second argument of preg_match is deprecated.

src/Handler/ContentRangeUploadHandler.php

@@ -67,7 +67,7 @@ public function __construct(Request $request, $file, $config)
     {
         parent::__construct($request, $file, $config);
 
-        $contentRange = $this->request->header(self::CONTENT_RANGE_INDEX);
+        $contentRange = $this->request->header(self::CONTENT_RANGE_INDEX, '');
 
         $this->tryToParseContentRange($contentRange);
     }