Fixes following deprecation warnings during unit tests
Deprecated: strtolower(): Passing null to parameter #1 ($string) of type string is deprecated in /var/www/vendor/intervention/image/src/Intervention/Image/Gd/Font.php on line 232
Deprecated: strtolower(): Passing null to parameter #1 ($string) of type string is deprecated in /var/www/vendor/intervention/image/src/Intervention/Image/Gd/Font.php on line 243
Issue https://github.com/silverstripe/silverstripe-framework/issues/10250
Fixes following deprecation warnings during unit tests