schemasalad:null being detected as items in an array

[MarkRobbo]

When the following syntax is used to denote the types of items in an array:

type:
    items:
    - 'null'
    - string
    type: array

The type is detected as: https://w3id.org/cwl/salad#null[]

array<CWLType> is listed as being supported by the spec which implies that we should be parsing more than one type when we currently do not.

It is unclear to the author as to the implication of null being within this array (makes the whole array optional as a consequence due to the possibility of an array of nothing?). Asked for clarification here.

Example: http://view.commonwl.org/workflows/github.com/bcbio/bcbio_validation_workflows/blob/master/NA12878-chr20/NA12878-platinum-chr20-workflow/main-NA12878-platinum-chr20.cwl

Branch here contains a naive approach blocking null from appearing as the type of arrays (in this case displaying the above example as just String[]) however this is not an ideal fix.