Don't return association if table is nil

[CGA1123]

Returning an association here where table == nil will result in a subsequent undefined method '[]' for nil:NilClass being thrown in get_attribute due to Ransack::Adapters::ActiveRecord::context#table_for returning nil.

Falling back to build_association works as expected.

Related #947

[gregmolnar]

@CGA1123 Thanks for your contribution. I cherry picked your commit and added a spec as well in another PR so closing this.