[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.
Also, quick question: Is the FQCN ok or should I add a `use` statement at the top of the file?