[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.
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