Scan error when scanning partial ABOUT file

The --package scan of a file name foo.ABOUT with:
`download_url: https://raw.githubusercontent.com/docker/docker/ff2de8dace1ba1c1f5e8542790ef5cd564375934/image/spec/v1.1.md`
fails to scan with:
```
scan_errors:
- |
ERROR: for scanner: packages:
ERROR: Unknown error:
Traceback (most recent call last):
File "/scancode-toolkit-master2/src/scancode/interrupt.py", line 91, in interruptible
return NO_ERROR, func(*(args or ()), **(kwargs or {}))
File "/scancode-toolkit-master2/src/scancode/api.py", line 344, in get_package_manifests
return dict(package_manifests=[
File "/scancode-toolkit-master2/src/scancode/api.py", line 345, in
package_manifests.to_dict()
AttributeError: 'NoneType' object has no attribute 'to_dict'
```