Support sorting by term weight for aggregated fields

Fixed little bug if $field_configurations contains no key fields (for 2.x-dev).

Unfortunalety I cannot test against 2.x (only 3.x is available). So I created a MR of existing code.
The code is running on a complex drupal since several months without problems.
Many thanks to @andy_w

While I think this is interesting functionality, we're currently not at the stage where we want to add more to the facets module, especially without rigorous testing. I think we should create this as a contrib extension instead, we can link to it from the readme.