[5.0] Fix Broken File Upload When Submitting A Form Without Selecting A File

This is my attempt at fixing (my first PR) the issue:
[5.0] File upload is broken when submitting a form without selecting a file #6189

Right now I'm overriding Symfony's Request::duplicate function so that it can handle empty file form inputs. Another solution is included in the issue (#6189), which modifies the createFromBase function. Both filter the input array to remove any files that are null.
Yeh, this needs to be imported.