Argument with prefix = empty string and separate = false is omitted from cmd line

## Expected Behavior
Argument with `prefix: ""` and `separate: false` should be in the command line.
Command line should be `echo non_existing_app`.

## Actual Behavior
Argument is not in the command line.
Command line is `non_existing_app`.

## Workflow Code
```
class: CommandLineTool
cwlVersion: v1.0
baseCommand: []
inputs:
echo: boolean
outputs: []
arguments:
- position: 1
shellQuote: false
prefix: ''
valueFrom: "non_existing_app"
- position: 0
prefix: ''
separate: false
shellQuote: false
valueFrom: "echo"
requirements:
- class: ShellCommandRequirement
```

## Your Environment
* cwltool version: 1.0.20190621234233
works fine with cwltool version: 1.0.20190228155703