InitialWorkDirRequirement fails to create an empty listing.entryname file

## Expected Behavior

empty listing.entryname file is expected to be created when inputs.annotation is NULL

## Actual Behavior

empty listing.entryname file is not created

## Workflow Code
```
cwlVersion: v1.0
label: 'test_JS'
# internal NCBI location for our reference: https://bitbucket.ncbi.nlm.nih.gov/users/badrazat/repos/jira/commits/a391294d18cf9d88168f61608eea591a230669a3#vc-PGAPX-313-Unable-to-open-the-manifest-for-/test_cwltool.cwl

class: CommandLineTool
requirements:
- class: InlineJavascriptRequirement
- class: InitialWorkDirRequirement
listing:
- entryname: annotation.mft
entry: ${ if ( inputs.annotation == null ) { return ''; } else { return inputs.annotation.path; } }

baseCommand: cat
inputs:
annotation:
type: File?
annotation_manifest:
type: string
default: annotation.mft
inputBinding:
position: 1
outputs: []

```

## Full Traceback
```
$ cwltool --debug --tmpdir-prefix ./tmpdir/ --leave-tmpdir --tmp-outdir-prefix ./tmp-outdir/ --copy-outputs --outdir ./outdir test_cwltool.cwl
/home/badrazat/tmp2/cwltool-git/cwltool/venv/bin/cwltool 1.0.20190228155703
Resolved 'test_cwltool.cwl' to 'file:///home/badrazat/bitbucket/repos/jira/vc-PGAPX-313-Unable-to-open-the-manifest-for-/test_cwltool.cwl'
Parsed job order from command line: {
"id": "test_cwltool.cwl",
"annotation": null,
"annotation_manifest": "annotation.mft"
}
[job test_cwltool.cwl] initializing from file:///home/badrazat/bitbucket/repos/jira/vc-PGAPX-313-Unable-to-open-the-manifest-for-/test_cwltool.cwl
[job test_cwltool.cwl] {
"annotation": null,
"annotation_manifest": "annotation.mft"
}
[job test_cwltool.cwl] path mappings is {}
[job test_cwltool.cwl] command line bindings is [
{
"position": [
-1000000,
0
],
"datum": "cat"
},
{
"position": [
1,
"annotation_manifest"
],
"datum": "annotation.mft"
}
]
[job test_cwltool.cwl] initial work dir {}
[job test_cwltool.cwl] /home/badrazat/bitbucket/repos/jira/vc-PGAPX-313-Unable-to-open-the-manifest-for-/tmp-outdir/bg7ji6s1$ cat \
annotation.mft
cat: annotation.mft: No such file or directory
Could not collect memory usage, job ended before monitoring began.
[job test_cwltool.cwl] completed permanentFail
[job test_cwltool.cwl] {}
[job test_cwltool.cwl] Removing input staging directory /tmp/tmp4w43_qvl
{}
Final process status is permanentFail

```

## Your Environment
* cwltool version:
1.0.20190228155703
* previous version that works correctly:
1.0.20181217162649