[FIX] Gracefully handle empty input_spec arg passed to Workflow init

[tclose]

This PR handles the case where an empty input_spec is provided to a Workflow init by raising a ValueError. Otherwise, the init enters an infinite recursion loop attempting to access the input spec.

Acknowledgment

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

Types of changes

• [ x ] 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

Handles the case when an empty input_spec is provided to Workflow init by raising an exception

Checklist

• [ x ] All tests passing
• [ x ] I have added tests to cover my changes
• I have updated documentation (if necessary)
• [ x ] 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 #497 (de03e10) into master (35c16f6) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #497      +/-   ##
==========================================
+ Coverage   83.01%   83.04%   +0.02%     
==========================================
  Files          20       20              
  Lines        4045     4046       +1     
  Branches     1118     1118              
==========================================
+ Hits         3358     3360       +2     
  Misses        491      491              
+ Partials      196      195       -1     

Flag	Coverage Δ
unittests	82.97% <100.00%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
pydra/engine/core.py	89.28% <100.00%> (+0.18%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 35c16f6...de03e10. Read the comment docs.

[djarecka]

@tclose - Thank you! Would you like to add your name to the zenodo file? https://github.com/nipype/pydra/blob/master/.zenodo.json