Adds Order#order_email to show the order's email

`spree_orders` table has the column `email` which stores the email of
guest orders, this change intends to look first at this property then
fallback to user's email.

admin/app/components/solidus_admin/orders/index/component.rb

@@ -143,7 +143,7 @@ def customer_column
       col: { class: "w-[400px]" },
       header: :customer,
       data: ->(order) do
-        customer_email = order.user&.email
+        customer_email = order.order_email
         content_tag :div, String(customer_email)
       end
     }

admin/spec/features/orders/index_spec.rb

@@ -11,6 +11,7 @@
     visit "/admin/orders"
     click_on "In Progress"
 
+    expect(page).to have_content("[email protected]")
     expect(page).to have_content("R123456789")
     expect(page).to have_content("$19.99")
     expect(page).to be_axe_clean

core/app/models/spree/order.rb

@@ -726,6 +726,10 @@ def validate_payments_attributes(attributes)
       end
     end
 
+    def order_email
+      email.presence || user&.email
+    end
+
     private
 
     def process_payments_before_complete

core/spec/models/spree/order_spec.rb

@@ -1832,4 +1832,26 @@ def validate(line_item)
       expect { subject }.to change { Spree::OrderPromotion.count }.from(1).to(0)
     end
   end
+
+  describe "order_email" do
+    context "when a new order is created" do
+      it "returns nil" do
+        expect(Spree::Order.new.order_email).to eq(nil)
+      end
+    end
+
+    context "when a logged in user creates an order" do
+      it "returns the customer's email" do
+        order = build(:order, user: build(:user, email: "[email protected]"))
+        expect(order.order_email).to eq("[email protected]")
+      end
+    end
+
+    context "when order email is different than user's email" do
+      it "gives preference to order's email" do
+        order = build(:order, email: "[email protected]", user: build(:user, email: "[email protected]"))
+        expect(order.order_email).to eq("[email protected]")
+      end
+    end
+  end
 end