Steps to reproduce the issue

When file uploads are optional, isSafeFile() still tries to check files that haven't been uploaded.
This is more obvious when using PHP 8 since fopen($path) will throw an error if $path is empty.
The only reason the below steps are needed is to easily illustrate the issue since I couldn't find a single place in Joomla! that has optional uploads.

• Edit administrator/components/com_media/views/tmpl/default.php
• Search for <input required type="file" id="upload-file" name="Filedata[]" multiple />
• Remove the required attribute so that it reads <input type="file" id="upload-file" name="Filedata[]" multiple />
• Additionally you can simply change the required attribute by using the Inspector in your browser
• Go to Content > Media
• Click on Upload on the toolbar
• Don't select a file
• Click on Start Upload

Expected result

Since no upload takes place, the page should refresh with an enqueued message.

Actual result

Error
An error has occurred.

0 Path cannot be empty
JPATH_ROOT\libraries\src\Filter\InputFilter.php:630 

System information (as much as possible)

PHP 8.0.1

Additional comments

Error comes from this line in libraries\src\Filter\InputFilter.php:

$fp = @fopen($tempName, 'r');`

The $_FILES array is populated, but tmp_name will be an empty string and error will be set to 4 which is UPLOAD_ERR_NO_FILE. The logic in isSafeFile() will still attempt to fopen() this empty string which leads to an error in PHP 8. Easiest fix would be:

$fp = strlen($tempName) ? @fopen($tempName, 'r') : false;

A more elaborate fix would be to also check the error segment of the $_FILES array.