partnerID in s_order should be nullable

partnerID should be nullable to reflect s_order_basket and make it more doctrine orm friendlier. its a foreign key on an empty string right now, there are also some other sideeffects. "migrations" not provided by pull request...
Please do not alter the `_sql/install/latest.sql` file directly.
Instead add a migration file to `_sql/migrations/` that does an alter table statement.

During build/deploy time we import `_sql/install/latest.sql` and then apply the migrations from `_sql/migrations/` on top.

To apply the migrations you can use the `build/ApplyDeltas.php` script or run `ant build-unit` inside the build directory.