prevent exceptions when incorrect geo entity types set sent

A NoMethodError occurred in geo_entities#index:

  undefined method `empty?' for nil:NilClass
  app/controllers/checklist/geo_entities_controller.rb:16:in `index'


-------------------------------
Request:
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  * URL        : http://sapi.unepwcmc-013.vm.brightbox.net/checklist/geo_entities?geo_entity_types_set=1?
  * HTTP Method: GET
  * IP address : 193.5.216.100
  * Parameters : {"geo_entity_types_set"=>"1?", "action"=>"index", "controller"=>"checklist/geo_entities"}
  * Timestamp  : 2014-02-28 09:43:00 UTC
  * Server : unepwcmc-013.vm.brightbox.net
  * Rails root : /home/rails/sapi/releases/20140228075541
  * Process: 8330