Steps to recreate

1. Start blacklight locally
2. Go to http://localhost:3000/?clause[0][field]=not_a_real_field&clause[0][query]=cats
3. Note that you get NoMethodError (undefined method 'display_label' for nil:NilClass)

Every other part of the stack is able to handle cases in which the field does not exist, I think that this class should match.

Alternatively, if we do want to throw an error in cases that the user tries to search a non-existant field, I think it should be a more intuitive error in a different part of the stack.