As per the PHP manual:
> If the `component` parameter is omitted, an associative array is returned.
> If the `component` parameter is specified, `parse_url()` returns a string (or an int, in the case of `PHP_URL_PORT`) instead of an array. If the requested component doesn't exist within the given URL, `null` will be returned.
Reference: [https://www.php.net/manual/en/function.parse-url.php#refsect1-function.parse-url-returnvalues PHP Manual: parse_url(): Return Values]
In this case, `parse_url()` is called with the `PHP_URL_PATH` as `$component`. This will return `null` in the majority of cases, as – exсept for subdirectory-based sites – `home_url()` returns a URL without the trailing slash, like `http://example.org`.
The return value of `parse_url()` was subsequently passed to `trim()`, leading to a `trim(): Passing null to parameter #1 ($string) of type string is deprecated` notice on PHP 8.1.
Fixed by adjusting the logic flow to:
* Only pass the return value of `parse_url()` to follow-on functions if it makes sense, i.e. if it isn't `null`, nor an empty string.
* Preventing calls to `preg_replace()` and `trim()` further down in the function logic flow, when `preg_replace()`/`trim()` would have nothing to do anyhow.
Follow-up to [25617].
Props jrf, hellofromTonya, SergeyBiryukov.
See #53635.
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