Currently scoping by taxonomies is broken.

Before this commit: https://github.com/theforeman/foreman/commit/35ed04f95f14c934ffebc874e90a3a4a6684fe71

`scope = scope.where(:id => taxable_ids) if taxable_ids`

did STILL call `scope.where(:id => [])` when taxable_ids were not found.
That returns an empty scope, and is the appropriate thing to do if there are no taxable IDs.
However after the commit, `if taxable_ids.present?` will make scope return `1=1` for the case where there are no taxable IDs.

If there are some taxable IDs, it does the right thing and scopes the object properly. That use case works fine.