Defend against nils in variant serializer

If unit_price.denominator ever returns nil, the serializer won't explode now.

app/serializers/api/variant_serializer.rb

@@ -41,7 +41,7 @@ def thumb_url
   end
 
   def unit_price_price
-    price_with_fees / unit_price.denominator
+    price_with_fees / (unit_price.denominator || 1)
   end
 
   def unit_price_unit

spec/serializers/api/variant_serializer_spec.rb

@@ -25,4 +25,22 @@
         :tag_list # Used to apply tag rules
       )
   end
+
+  describe "#unit_price_price" do
+    context "without fees" do
+      it "displays the price divided by the unit price denominator" do
+        allow(subject).to receive_message_chain(:unit_price, :denominator) { 1000 }
+
+        expect(subject.unit_price_price).to eq(variant.price / 1000)
+      end
+    end
+
+    context "when the denominator returns nil" do
+      it "returns the price" do
+        allow(subject).to receive_message_chain(:unit_price, :denominator) { nil }
+
+        expect(subject.unit_price_price).to eq(variant.price)
+      end
+    end
+  end
 end