Wrong rdfs namespace in cwltool --print-rdf

## Expected Behavior

cwltool --print-rdf should use conventional valid namespaces where they exist.

## Actual Behavior

Wrong prefix for the `rdfs` namespace, seems to be caused by `` being interpreted as an URI instead of a qname mapped to the rdfs namespace `http://www.w3.org/2000/01/rdf-schema# `

```turtle
@prefix ns1: .
```

## Workflow Code

https://rawgit.com/common-workflow-language/workflows/master/workflows/hello/hello.cwl

## Full Traceback

cwltool --print-rdf https://rawgit.com/common-workflow-language/workflows/master/workflows/hello/hello.cwl

```turtle
@prefix SoftwareRequirement: .
@prefix Workflow: .
@prefix cwl: .
@prefix ns1: .
@prefix rdf: .
@prefix rdfs: .
..
cwl:default "Hello World" ;
cwl:inputBinding [ CommandLineBinding:position 1 ],
[ CommandLineBinding:position 1 ] ;
sld:type xsd:string ;
ns1:comment "The message to print" .
```

## Your Environment
* cwltool version: 1.0.20170525215327