ci-license-scan: Treat empty license as unknown

Due to an upstream bug scan-copyrights reports license as empty in some
cases. Workaround this issue by treating empty license in the same way
unknown license are treated.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>

package-source-builder/overlay/usr/bin/ci-license-scan

@@ -400,7 +400,7 @@ def main():
         for p in c.all_files_paragraphs():
             if p.license:
                 for f in p.files:
-                    if p.license.synopsis == 'UNKNOWN':
+                    if p.license.synopsis in ['UNKNOWN', '']:
                         copyright, license = detect_license_for(f, debian_copyrights)
                         if copyright is not None and license is not None:
                             if license.synopsis.rstrip('+').lower() in disallowlist: