Add an empty product to blank invoice form

There is no point in creating a new invoice without products, thus we
were forcing users to always use the “Add product” button for no reason
other than it was easier for me….

I wanted to add the product inside ServeInvoice, when the slug is “new”,
but then it tried to compute the invoice total without price or quantity
and it failed.  Thus, i add that product after it has done the
computation query.

pkg/invoices.go

@@ -413,6 +413,9 @@ func newNewInvoicePage(form *invoiceForm, r *http.Request) *newInvoicePage {
 	if err != nil {
 		panic(err)
 	}
+	if len(form.Products) == 0 {
+		form.Products = append(form.Products, newInvoiceProductForm(0, company, getLocale(r), mustGetTaxOptions(r.Context(), conn, company)))
+	}
 	return page
 }