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]
While it is probably unlikely that someone would have a direct link to the plugin/theme editor on their home page or even on someone else's homepage, it is entirely possible for the referrer URL to not have a "path" component.
In PHP 8.1, this would lead to a `basename(): Passing null to parameter #1 ($string) of type string is deprecated` notice.
Changing the logic around and adding validation for the return type value of `parse_url()` prevents that.
Follow-up to [51606], [51622], [51626].
Props jrf, hellofromTonya, SergeyBiryukov.
See #53635.
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