Return an error for facet parameters with empty values
Return an error for facet parameters with empty values

If a user requests a facet parameter with an empty value (for example /search.json?facet_policies) then rummager currently throws an exception when attempting to split the nil value.

This commit adds a check to change the nil value to an empty string, allowing the rest of the function to complete and display an error message.

alphagov/rummager,alphagov/rummager