Fix rspec

app/models/subscriber_inventory.rb

@@ -37,7 +37,7 @@ def self.select_filtered_plan_ids(filters = {})
     if filters.has_key?(:year)
       filter_criteria[:year] = filters[:year]
     end
-    return nil if filter_criteria.empty?
+    return [] if filter_criteria.empty?
 
     Rails.cache.fetch("plan-ids-#{filter_criteria[:hios_plan_id]}-#{filter_criteria[:year]}", expires_in: 24.hour) do
       plans = Plan.where(filter_criteria)

spec/models/subscriber_inventory_spec.rb

@@ -79,7 +79,7 @@
     it "returns the coverage history in the format ACA Entities expects" do
       expect(Generators::CoverageInformationSerializer).to receive(
         :new
-      ).with(person, nil).and_return(coverage_information_serializer)
+      ).with(person, []).and_return(coverage_information_serializer)
       expect(SubscriberInventory.coverage_inventory_for(person)).to eq({})
     end
   end