[5.x] Fix files fieldtype

[duncanmcclean]

This pull request fixes an issue with the Files Fieldtype (which powers the "Re-upload asset" action).

In #10272, we dropped the default values from various fieldtypes, including the Files fieldtype, since the default values weren't really needed.

This caused an issue with the Files fieldtype which we attempted to fix in #10333, however, that PR only fixed the rendering of the fieldtype. You'd get another error related to the value not being set after uploading a file.

This PR fixes it by reverting the fix from #10333, and instead returning an empty array in the fieldtype's preProcess method so that value is never null.

Fixes #10440.

[andjsch]

Hey @duncanmcclean
By accident, I stumbled upon an related issue today, regarding #10272. I do have a private API to fetch form submissions, which causes an error (but only on newly created forms > only there the config fields are missing) while calling 'visibility' => $field->config()['visibility'].

What I do wonder now: should calling $field->config() merge the defaults? It doesn't right now.

[jasonvarga]

Please open a separate issue.