guard against nil @associations_pot

I was getting the following error while attempting to upgrade to Rails 4.1:

```
NoMethodError: undefined method `[]' for nil:NilClass
from ~/.rbenv/versions/2.1.0/lib64/ruby/gems/2.1.0/bundler/gems/ransack-d51c78f9071f/lib/ransack/adapters/active_record/context.rb:169:in `block in build_or_find_association'
```

I added a simple guard clause to the condition, since it appears that `@associations_pot` is simply never set until after the first run through this method, though I am not entirely clear on the details.  This seems to fix everything, without breaking the test suite, and my usage of the gem still works.