Unknowns in widnows manifests not relevant?

There are a lot of windows-type manifests (like type: `winexe`) detected in SCTK which have extracted license statements like the following:
```
{'LegalCopyright': 'Copyright (C) Microsoft Corporation. All rights reserved.', 'LegalTrademarks': '', 'License': None}
```
And these are detected as: ``"declared_license_expression": "unknown",``
Here's an example in our test data: https://github.com/nexB/scancode-toolkit/blob/develop/tests/packagedcode/data/plugin/com-package-expected.json#L41, more are present.
Since this is a copyright and then no license is specified, is this really an Unknown license?
Should we just ignore if we have a `'License': None` here and have `None` in the license fields instead?

@pombredanne @JonoYang RFC