Fix debian/copyright format for empty-short-license-in-dep5-copyright

Lintian reports empty-short-license-in-dep5-copyright warnings.
The License field in debian/copyright should not be empty by dep5 standard.
This commit adds the missing empty field.

Signed-off-by: Ying-Chun Liu (PaulLiu) <paulliu@debian.org>
Merge pull request #860 from grandpaul/fix-copyright-format

Fix debian/copyright format for empty-short-license-in-dep5-copyright