fix empty scatter bug

[mr-c]

fixes #1668

• tests
  why did no conformance test trigger this? Scatter inputs in CWL v1.[012] must be arrays, not empty; so allowing this requires a change to the CWL standards

See #1668 (comment)

[codecov]

Codecov Report

Merging #1669 (86c6f8e) into main (8763bed) will increase coverage by 0.59%.
The diff coverage is 85.71%.

❗ Current head 86c6f8e differs from pull request most recent head db82bbb. Consider uploading reports for the commit db82bbb to get more accurate results

@@            Coverage Diff             @@
##             main    #1669      +/-   ##
==========================================
+ Coverage   66.43%   67.02%   +0.59%     
==========================================
  Files          89       89              
  Lines       15908    15908              
  Branches     4216     4216              
==========================================
+ Hits        10569    10663      +94     
+ Misses       4240     4154      -86     
+ Partials     1099     1091       -8     

Impacted Files	Coverage Δ
cwltool/workflow_job.py	76.88% <80.00%> (+17.11%)	⬆️
cwltool/executors.py	82.58% <100.00%> (ø)
cwltool/job.py	79.88% <100.00%> (ø)
cwltool/cwltool/workflow_job.py	81.11% <0.00%> (ø)
cwltool/cwltool/job.py	62.25% <0.00%> (+0.79%)	⬆️
cwltool/workflow.py	87.64% <0.00%> (+1.99%)	⬆️
cwltool/checker.py	89.78% <0.00%> (+3.40%)	⬆️

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