import: utils: 'alist->package' allows false license.

Reported by <pkill9@runbox.com>.
Fixes <https://bugs.gnu.org/30470>.

* guix/import/utils.scm (alist->package): Check whether 'license' is
false and set the 'license' field to #f in this case.
* tests/import-utils.scm ("alist->package with false license"): New
test.