Error if prefix is absent

Hello!
If there is no "prefix" in workflow.step.run.inputBinding:
```
Traceback (most recent call last):
File "/usr/local/lib/python2.7/dist-packages/cwltool/workflow.py", line 587, in job
**kwargs):
File "/usr/local/lib/python2.7/dist-packages/cwltool/draft2tool.py", line 382, in job
j.command_line = flatten(map(builder.generate_arg, builder.bindings))
File "/usr/local/lib/python2.7/dist-packages/cwltool/builder.py", line 198, in generate_arg
args.append(prefix + self.tostr(j))
TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'
[step varscan] Cannot make job: unsupported operand type(s) for +: 'NoneType' and 'str'
```