packages: 'package-field-location' handles 'search-path' returning #f.

Fixes <https://bugs.gnu.org/46390>.
Reported by zimoun <zimon.toutoune@gmail.com>.

This is similar to the fix in d10474c3.

* guix/packages.scm (package-field-location): Handle FILE not in %LOAD-PATH.
* tests/guix-lint.sh: Add test.