Add missing property, fix call to htmlspecialchars by ensuring the pa…

Fix two deprecated warnings:

PHP Deprecated:  Creation of dynamic property XLSXWriter::$tempdir is deprecated in vendor/mk-j/php_xlsxwriter/xlsxwriter.class.php on line 44

PHP Deprecated:  htmlspecialchars(): Passing null to parameter #1 ($string) of type string is deprecated in vendor/mk-j/php_xlsxwriter/xlsxwriter.class.php on line 760