Find approximate tax rate if no other is found

There are several cases in which a tax rate is not associated with an
adjustment. We find the closest one for reporting here. It is not a good
solution, but a workaround introduced for reporting:
https://github.com/openfoodfoundation/openfoodnetwork/pull/1496

app/services/tax_rate_finder.rb

@@ -4,6 +4,13 @@
 class TaxRateFinder
   # @return [Array<Spree::TaxRate>]
   def tax_rates(originator, source, amount, included_tax)
+    find_associated_tax_rate(originator, source) ||
+      find_closest_tax_rates_from_included_tax(amount, included_tax)
+  end
+
+  private
+
+  def find_associated_tax_rate(originator, source)
     case originator
     when Spree::TaxRate
       [originator]
@@ -15,13 +22,9 @@ class TaxRateFinder
       when Spree::Order
         originator.tax_category ? originator.tax_category.tax_rates.match(source) : []
       end
-    else
-      find_closest_tax_rates_from_included_tax(amount, included_tax)
     end
   end
 
-  private
-
   # shipping fees and adjustments created from the admin panel have
   # taxes set at creation in the included_tax field without relation
   # to the corresponding TaxRate, so we look for the closest one

spec/services/tax_rate_finder_spec.rb

@@ -51,7 +51,7 @@ describe TaxRateFinder do
     # There is a bug that leaves orphan adjustments on an order after
     # associated line items have been removed.
     # https://github.com/openfoodfoundation/openfoodnetwork/issues/3127
-    pending "deals with a missing line item" do
+    it "deals with a missing line item" do
       rates = TaxRateFinder.new.tax_rates(
         enterprise_fee,
         nil,