null wrongly requires quoting

[arunisaac]

Here, I have a simple workflow in which the input message is optional. The type is specified as an array of string and null.

cwlVersion: v1.2
class: CommandLineTool
baseCommand: echo
inputs:
  message:
    type:
      - string
      - null
    inputBinding:
      position: 1
outputs: {}

When I run this workflow using cwltool, I get the following validation error:

$ cwltool foo.cwl
ERROR Tool definition failed validation:
foo.cwl:1:1: Object 'foo.cwl' is not valid because
               tried 'CommandLineTool' but
foo.cwl:4:1:     the 'inputs' field is not valid because
foo.cwl:5:3:       item is invalid because
foo.cwl:6:5:         the 'type' field is not valid because
                       tried array of <CWLType or CommandInputRecordSchema or CommandInputEnumSchema or CommandInputArraySchema or
                       string> but
                         item is invalid because
                           - tried CWLType but
                             value is a NoneType but expected a string
                           - tried CommandInputRecordSchema but
                             is not a dict. Expected a CommandInputRecordSchema object.
                           - tried CommandInputEnumSchema but
                             is not a dict. Expected a CommandInputEnumSchema object.
                           - tried CommandInputArraySchema but
                             is not a dict. Expected a CommandInputArraySchema object.
                           - tried string but
                             the value is not string

However, if I double quote the null in the type (line 8), the workflow validates normally. I suppose this is cwltool getting confused about YAML's type ambiguity. The correct behaviour in this context would be to treat null as the string "null".