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

[Watkinsong]

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

[ocrow]

These two fixes are required for the library to run without warnings under PHP 8.
PHP 7 is now in end of life status.
Requesting this commit be incorporated in a release of the library.

[w263756314]

support