Don't try to parse empty phone number fields

frontend/src/Datarekisteri/Frontend/FormFields.hs

@@ -33,6 +33,7 @@ telephoneField = Field
     { fieldParse = \rawValues _ ->
         case rawValues of
           [] -> pure $ Right Nothing
+          [""] -> pure $ Right Nothing
           [x] -> pure $ maybe (Left "could not parse as a phone number") (Right . Just) $ toPhoneNumber x
           _ -> pure $ Left $ "Expected one value"
     , fieldView = \id name otherAttributes result isRequired ->