Fix deprecation warning on empty str_replace() parameter

[jhuhta]

This fixes a deprecation error:

Deprecated: str_replace(): Passing null to parameter #3 ($subject) of type array|string is deprecated in /var/www/html/vendor/h5p/h5p-core/h5p.classes.php on line 1763

The error is given in Drupal on PHP 8.1 when saving a node with a H5P element.

[pjotrsavitski]

Thumbs up for the refactoring of str_replace functions enclosed into one another, although the code could be even more concise if you remove the empty check and just use the $value ?? '' instead. It will still be as readable and give the same result.