Fix PHP8.1 issue passing NULL to sha1()

[smulvih2]

After moving to PHP8.1 I am getting the following error when a field is empty:

Deprecated function: sha1(): Passing null to parameter #1 ($string) of type string is deprecated in DaveChild\TextStatistics\Text::cleanText() (line 34 of /var/www/vendor/davechild/textstatistics/src/DaveChild/TextStatistics/Text.php).

This PR checks if the string in question is NULL for proper handling.