Update Spree::Price parsing to match LocalizedNumber.parse

Spree::Price parsing was returning 0.0 when given a an empty string as
price, resulting in a variant being valid even if no price was given. It
only happened if `Spree::LocalizedNumber` wasn't used.
Spree::LocalizedNumber` return nil if given a blank number.

app/models/spree/price.rb

@@ -38,6 +38,7 @@ module Spree
 
     # strips all non-price-like characters from the price, taking into account locale settings
     def parse_price(price)
+      return nil if price.blank?
       return price unless price.is_a?(String)
 
       separator, _delimiter = I18n.t([:'number.currency.format.separator',

spec/models/spree/price_spec.rb

@@ -34,5 +34,25 @@ module Spree
         expect(variant.reload.price).to eq 10.25
       end
     end
+
+    describe "#price=" do
+      subject { Spree::Price.new }
+
+      context "with a number" do
+        it "returns the same number" do
+          subject.price = 12.5
+
+          expect(subject.price).to eq(12.5)
+        end
+      end
+
+      context "with empty string" do
+        it "sets the price to nil" do
+          subject.price = ""
+
+          expect(subject.price).to be_nil
+        end
+      end
+    end
   end
 end