Prevent PHP Warning for Missing Request Parameter

Updated the code to avoid PHP warnings when the request parameter is missing. Now using the null coalescing operator (`?? 0`) to provide a default value, preventing undefined index errors. Also, removed the redundant empty check.

Ref: https://github.com/humanmade/smart-media/blob/master/inc/cropper/namespace.php#L560-L565

Inspect inc/cropper/namespace.php around lines 560-565, where the missing request parameter is handled. Confirm the parameter defaults to 0 without an undefined-index warning and that the redundant empty check is removed; verify the resulting behavior with the relevant project checks.

Written by the indexing model from the issue text.