Ensure TaxRateFinder returns an array if the given adjustment has no tax

app/services/tax_rate_finder.rb

@@ -6,15 +6,12 @@
 class TaxRateFinder
   # @return [Array<Spree::TaxRate>]
   def self.tax_rates_of(adjustment)
-    new.tax_rates(
-      adjustment.originator,
-      adjustment.adjustable
-    )
+    new.tax_rates(adjustment.originator, adjustment.adjustable)
   end
 
   # @return [Array<Spree::TaxRate>]
   def tax_rates(originator, adjustable)
-    find_associated_tax_rate(originator, adjustable)
+    find_associated_tax_rate(originator, adjustable) || []
   end
 
   private

spec/services/tax_rate_finder_spec.rb

@@ -5,7 +5,9 @@ require 'spec_helper'
 describe TaxRateFinder do
   describe "getting the corresponding tax rate" do
     let(:amount) { BigDecimal(120) }
-    let(:tax_rate) { create_rate(0.2) }
+    let(:tax_rate) {
+      create(:tax_rate, amount: 0.2, calculator: Calculator::DefaultTax.new, zone: zone)
+    }
     let(:tax_category) { create(:tax_category, tax_rates: [tax_rate]) }
     let(:zone) { create(:zone_with_member) }
     let(:shipment) { create(:shipment) }
@@ -13,35 +15,36 @@ describe TaxRateFinder do
     let(:enterprise_fee) { create(:enterprise_fee, tax_category: tax_category) }
     let(:order) { create(:order_with_taxes, zone: zone) }
 
+    subject { TaxRateFinder.new }
+
     it "finds the tax rate of a shipping fee" do
-      rates = TaxRateFinder.new.tax_rates(tax_rate, shipment)
+      rates = subject.tax_rates(tax_rate, shipment)
       expect(rates).to eq [tax_rate]
     end
 
     it "deals with soft-deleted tax rates" do
       tax_rate.destroy
-      rates = TaxRateFinder.new.tax_rates(tax_rate, shipment)
+      rates = subject.tax_rates(tax_rate, shipment)
       expect(rates).to eq [tax_rate]
     end
 
     it "finds the tax rate of an enterprise fee" do
-      rates = TaxRateFinder.new.tax_rates(enterprise_fee, order)
+      rates = subject.tax_rates(enterprise_fee, order)
       expect(rates).to eq [tax_rate]
     end
 
     it "deals with a soft-deleted line item" do
       line_item.destroy
-      rates = TaxRateFinder.new.tax_rates(enterprise_fee, line_item)
+      rates = subject.tax_rates(enterprise_fee, line_item)
       expect(rates).to eq [tax_rate]
     end
 
-    def create_rate(amount)
-      create(
-        :tax_rate,
-        amount: amount,
-        calculator: Calculator::DefaultTax.new,
-        zone: zone
-      )
+    context "when the given adjustment has no associated tax" do
+      let(:adjustment) { create(:adjustment) }
+
+      it "returns an empty array" do
+        expect(subject.tax_rates(adjustment.originator, adjustment.adjustable)).to eq []
+      end
     end
   end
 end