fix: Allow copy with empty suffix

[fsbraun]

Description

The copy files and folders admin form allows for a suffix to be entered, which is placed in the filename of the copied file between the base name and the extension.

This PR allows for empty suffices, i.e. the standard case of no change in filename.

Related resources

• Copy file but leave empty suffix #1338 is fixed
• #...

Checklist

• I have opened this pull request against master
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #workgroup-pr-review on
  Slack to find a “pr review buddy” who is going to review my pull request.

[codecov]

Codecov Report

Merging #1386 (ce5957a) into master (71d16ca) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1386   +/-   ##
=======================================
  Coverage   73.98%   73.98%           
=======================================
  Files          75       75           
  Lines        3436     3436           
  Branches      552      552           
=======================================
  Hits         2542     2542           
  Misses        722      722           
  Partials      172      172           

Impacted Files	Coverage Δ
filer/admin/forms.py	68.29% <100.00%> (ø)