empty.yml can't be quite empty..?

Avoid bug:

```
Resolved 'expression.cwl' to 'file:///home/stain/src/cwl-test/expression.cwl'
Traceback (most recent call last):
  File "/home/stain/bin/cwl-runner", line 11, in <module>
    sys.exit(main())
  File "/home/stain/.local/lib/python2.7/site-packages/cwltool/main.py", line 701, in main
    fetcher_constructor=fetcher_constructor)
  File "/home/stain/.local/lib/python2.7/site-packages/cwltool/main.py", line 494, in load_job_order
    if "cwl:tool" in job_order_object:
TypeError: argument of type 'NoneType' is not iterable
```

v1.0/UserGuide.yml

@@ -662,10 +662,17 @@
     - |
       ```
       
-      As this tool does not require any `inputs` we can run it with an empty job file:
+      As this tool does not require any `inputs` we can run it with an (almost) empty job file:
+      
+      *empty.yml*
+      ```
+    - $include: examples/empty.yml
+    - |
+      ```
+      
+      We can then run `expression.cwl`:
 
       ```
-      $ touch empty.yml
       $ cwl-runner expression.cwl empty.yml
       [job 140000594593168] /home/example$ echo -A 2 -B baz -C 10 9 8 7 6 5 4 3 2 1
       -A 2 -B baz -C 10 9 8 7 6 5 4 3 2 1