"/dev/null" not accepted as a File path

```
$ cwltool 396625-2.cwl
INFO /home/michael/src/toil/env3.7/bin/cwltool 1.0.20190815141648
INFO Resolved '396625-2.cwl' to 'file:///home/michael/cwl-v1.1/396625-2.cwl'
ERROR Workflow error, try again with --debug for more information:
Input file file:///dev/null (at /dev/null) not found or is not a regular file.
```

``` cwl
cwlVersion: v1.0
class: CommandLineTool

inputs:
optional_file:
type: File
default:
class: File
path: /dev/null

baseCommand: [ls, -l]

outputs: []
```

I'm not convinced that this should be allowed, but the v1.{0,1} standards don't say that this isn't allowed.

This issue is inspired by https://www.biostars.org/p/396625/