Added support for empty arrays in restrictions.

This allows your timid code to go from looking like this
  with(:my_field, some_values) if some_values.present?
to become confident code like this
  with(:my_field, some_values)
Made empty array trigger a query equivalent to a no-op. Added documentation about the behaviour.
Merge pull request #334 from culturecode/empty-array-support

Added support for empty arrays in restrictions.