Fix Order#tax_address method

Calling `#tax_address` on an order that has no address but also no
store associates raises the following error:

    undefined method `default_cart_tax_location' for nil:NilClass

Given that the `store` association is optional, and/or it may not
have been already set on the `Spree::Order` instance, we should
avoid raising an error.

A simple way to expose the issue is by entering the following
command in the Rails console:

    Spree::Order.new.tax_address

core/app/models/spree/order.rb

@@ -255,7 +255,7 @@ def tax_address
  ship_address
  else
  bill_address
- end || store.default_cart_tax_location
+ end || store&.default_cart_tax_location
  end
 
  def updater

core/spec/models/spree/order_spec.rb

@@ -467,16 +467,32 @@ def merge!(other_order, user = nil)
  context "when tax_using_ship_address is true" do
  let(:tax_using_ship_address) { true }
 
- it 'returns the stores default cart tax location' do
- expect(subject).to eq(store.default_cart_tax_location)
+ context "when the order is associated with a store" do
+ it 'returns the stores default cart tax location' do
+ expect(subject).to eq(store.default_cart_tax_location)
+ end
+ end
+
+ context "when the order is not associated with a store" do
+ let(:store) { nil }
+
+ it { is_expected.to be_nil }
  end
  end
 
  context "when tax_using_ship_address is not true" do
  let(:tax_using_ship_address) { false }
 
- it 'returns the stores default cart tax location' do
- expect(subject).to eq(store.default_cart_tax_location)
+ context "when the order is associated with a store" do
+ it 'returns the stores default cart tax location' do
+ expect(subject).to eq(store.default_cart_tax_location)
+ end
+ end
+
+ context "when the order is not associated with a store" do
+ let(:store) { nil }
+
+ it { is_expected.to be_nil }
  end
  end
  end