adding an Attribute Error to the exception for get_value

[djarecka]

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

this should "fix" the issue from #577, if output is None we should enter the exception block

Checklist

• I have added tests to cover my changes (if necessary)
• I have updated documentation (if necessary)

[codecov]

Codecov Report

Base: 76.74% // Head: 76.79% // Increases project coverage by +0.04% 🎉

Coverage data is based on head (a12730d) compared to base (4d6a65b).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #592      +/-   ##
==========================================
+ Coverage   76.74%   76.79%   +0.04%     
==========================================
  Files          19       19              
  Lines        4339     4339              
  Branches     1192     1192              
==========================================
+ Hits         3330     3332       +2     
+ Misses        821      818       -3     
- Partials      188      189       +1     

Flag	Coverage Δ
unittests	76.69% <100.00%> (+0.04%)	⬆️

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%> (ø)
pydra/engine/helpers.py	80.61% <0.00%> (+0.47%)	⬆️

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