small fix copyfile_input

[djarecka]

Acknowledgment

• I acknowledge that this contribution will be available under the Apache 2 license.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Summary

• adding MultiInputFile as a possible type to copyfile_input (accepting list as a field)
• checking if the input is not NOTHING

Checklist

• All tests passing
• I have added tests to cover my changes
• I have updated documentation (if necessary)
• My code follows the code style of this project
  (we are using black: you can pip install pre-commit,
  run pre-commit install in the pydra directory
  and black will be run automatically with each commit)

[codecov]

Codecov Report

Merging #415 (6e66484) into master (6177c68) will decrease coverage by 0.12%.
The diff coverage is 46.15%.

@@            Coverage Diff             @@
##           master     #415      +/-   ##
==========================================
- Coverage   82.50%   82.37%   -0.13%     
==========================================
  Files          19       19              
  Lines        3847     3853       +6     
  Branches     1053     1055       +2     
==========================================
  Hits         3174     3174              
- Misses        483      488       +5     
- Partials      190      191       +1     

Flag	Coverage Δ
unittests	82.29% <46.15%> (-0.13%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pydra/engine/helpers_file.py	78.20% <46.15%> (-1.66%)	⬇️

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