PHP 8.1: fix "passing null to non-nullable" deprecation notice (#689)
PHP 8.1: fix "passing null to non-nullable" deprecation notice (#689)

As of PHP 8.1, passing `null` to non-nullable arguments of PHP native functions is deprecated.
Ref: https://wiki.php.net/rfc/deprecate_null_to_scalar_internal_arg

The `SimplePie_Enclosure::get_extension()` returns the file extension as a string or `null`.

In the `SimplePie_Enclosure::get_real_type()` method, the return value of a call to the `get_extension()` method was directly passed on to the PHP native `strtolower()` function, leading to the PHP 8.1 `strtolower(): Passing null to parameter #1 ($string) of type string is deprecated` deprecation notice.

This small change fixes this issue without BC break.

This is covered by 36 of the existing tests in the `OldTest` class.

Co-authored-by: jrfnl <50d644c256f2863fd67b233efe329d3f54f4e258@users.noreply.github.com>
simplepie/simplepie,mblaney/simplepie,FreshRSS/simplepie,simplepie/simplepie,FreshRSS/simplepie,mblaney/simplepie