Not dumping empty lists doesn't conform to the spec

[multimeric]

Currently, cwlgen.utils.should_exclude_object will cause the exporter to skip empty lists. This means that if you have an empty list of outputs, the output key will not be included in the resulting CWL. However the spec requires that the outputs list be present. This causes an empty-output tool to fail validation under cwltool.

[illusional]

Thanks for the report @TMiguelT, this is fixed and released in v0.4.1.