type checking: optional enum source with an optional string sink?

[mr-c]

https://github.com/datirium/workflows/blob/00ea05e22788029370898fd4c17798b11edf0e57/workflows/hopach.cwl

$ cwltool --validate https://github.com/datirium/workflows/raw/00ea05e22788029370898fd4c17798b11edf0e57/workflows/hopach.cwl
ERROR Tool definition failed validation:

hopach.cwl:66:5:   Source 'target_column'
                                  of type ["null",
                                  {"type": "enum",
                                  "symbols":
                                  ["https://github.com/datirium/workflows/raw/master/workflows/hopach.cwl#target_column/Rpkm",
                                  "https://github.com/datirium/workflows/raw/master/workflows/hopach.cwl#target_column/TotalReads"]}]
                                  is incompatible
hopach.cwl:302:7:   with sink
                                   'target_column' of
                                   type ["null",
                                   "string"]

Related: common-workflow-language/common-workflow-language#568