Fix : htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated

htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated

src/PhpWord/Escaper/Xml.php

@@ -27,6 +27,6 @@ class Xml extends AbstractEscaper
     protected function escapeSingleValue($input)
     {
         // todo: omit encoding parameter after migration onto PHP 5.4
-        return htmlspecialchars($input, ENT_QUOTES, 'UTF-8');
+        return (!is_null($input)) ? htmlspecialchars($input, ENT_QUOTES, 'UTF-8') : null;
     }
 }