Remove blank strings from array report parameters

Remove the blank strings that Rails multiple selects add by default to
make sure that blank lists are still submitted to the server as arrays
instead of nil.

https://api.rubyonrails.org/classes/ActionView/Helpers/FormOptionsHelper.html#method-i-select

lib/open_food_network/reports/parameters/base.rb

@@ -5,6 +5,7 @@ module OpenFoodNetwork
         extend ActiveModel::Naming
         extend ActiveModel::Translation
         include ActiveModel::Validations
+        include ActiveModel::Validations::Callbacks
 
         def initialize(attributes = {})
           attributes.each do |key, value|

lib/order_management/reports/enterprise_fee_summary/parameters.rb

@@ -15,6 +15,8 @@ module OrderManagement
         attr_accessor :start_at, :end_at, :distributor_ids, :producer_ids, :order_cycle_ids,
                       :enterprise_fee_ids, :shipping_method_ids, :payment_method_ids
 
+        before_validation :cleanup_arrays
+
         validates :start_at, :end_at, date_time_string: true
         validates :distributor_ids, :producer_ids, integer_array: true
         validates :order_cycle_ids, integer_array: true
@@ -41,6 +43,20 @@ module OrderManagement
 
           errors.add(:end_at, DATE_END_BEFORE_START_ERROR) unless start_at < end_at
         end
+
+        # Remove the blank strings that Rails multiple selects add by default to
+        # make sure that blank lists are still submitted to the server as arrays
+        # instead of nil.
+        #
+        # https://api.rubyonrails.org/classes/ActionView/Helpers/FormOptionsHelper.html#method-i-select
+        def cleanup_arrays
+          distributor_ids.reject!(&:blank?)
+          producer_ids.reject!(&:blank?)
+          order_cycle_ids.reject!(&:blank?)
+          enterprise_fee_ids.reject!(&:blank?)
+          shipping_method_ids.reject!(&:blank?)
+          payment_method_ids.reject!(&:blank?)
+        end
       end
     end
   end

spec/lib/order_management/reports/enterprise_fee_summary/parameters_spec.rb

@@ -21,6 +21,24 @@ describe OrderManagement::Reports::EnterpriseFeeSummary::Parameters do
       it { expect(subject).to validate_integer_array(:shipping_method_ids) }
       it { expect(subject).to validate_integer_array(:payment_method_ids) }
 
+      it "allows integer arrays to include blank string and cleans it up" do
+        subject.distributor_ids = ["", "1"]
+        subject.producer_ids = ["", "1"]
+        subject.order_cycle_ids = ["", "1"]
+        subject.enterprise_fee_ids = ["", "1"]
+        subject.shipping_method_ids = ["", "1"]
+        subject.payment_method_ids = ["", "1"]
+
+        expect(subject).to be_valid
+
+        expect(subject.distributor_ids).to eq(["1"])
+        expect(subject.producer_ids).to eq(["1"])
+        expect(subject.order_cycle_ids).to eq(["1"])
+        expect(subject.enterprise_fee_ids).to eq(["1"])
+        expect(subject.shipping_method_ids).to eq(["1"])
+        expect(subject.payment_method_ids).to eq(["1"])
+      end
+
       describe "requiring start_at to be before end_at" do
         let(:now) { Time.zone.now }