fix bug in change working directory impl

[toondaey]

There was an issue where if this argument is not provided, it crashes the extractor. This fixes it.

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 3 lines in your changes missing coverage. Please review.

Project coverage is 79.04%. Comparing base (015ce8a) to head (c8ed47d).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
cognite/extractorutils/unstable/core/runtime.py	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #459      +/-   ##
==========================================
+ Coverage   78.93%   79.04%   +0.10%     
==========================================
  Files          42       42              
  Lines        3650     3650              
==========================================
+ Hits         2881     2885       +4     
+ Misses        769      765       -4     

Files with missing lines	Coverage Δ
cognite/extractorutils/unstable/core/runtime.py	50.30% <57.14%> (+2.45%)	⬆️

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[rsjr]

risk review ok
Fixing a bug in a non-released feature