[9.x] Fix TrimStrings middleware with non-UTF8 characters

[MrMicky-FR]

Currently, the TrimStrings middleware sets to null all requests inputs with a unicode character not in UTF-8, which can be really frustrating to debug

It can be reproduced by using a POST request with a body like foo=abcd%E9: the foo input will just be null if the TrimString middleware is used.

This issue was introduced in #40600, because preg_replace with u flag will return null for a non-UTF8 input (with Malformed UTF-8 characters, possibly incorrectly encoded as preg_last_error_msg())

As a fix, when preg_replace return a null value, it's simply pass the value to the PHP trim function, which was the behavior used by Laravel for years. This doesn’t change the current behavior for valid UTF-8 inputs.

[MrMicky-FR]

Not sure why the tests are failing, as the failing test (Mockery\Exception\NoMatchingExpectationException in ThrottlesExceptionsWithRedisTest) seems unrelated to the changes in this PR ? And the tests was passing for the commit on my fork: https://github.com/MrMicky-FR/laravel-framework/commit/5076a1da69bc465797ff6ebe7957aca369df0f9f
EDIT: Tests are now all passing after restarting them with a dummy commit