Don't try to parse empty email fields

frontend/src/Datarekisteri/Frontend/FormFields.hs

@@ -17,6 +17,7 @@ emailField = Field
     { fieldParse = \rawValues _ ->
         case rawValues of
           [] -> pure $ Right Nothing
+          [""] -> pure $ Right Nothing
           [x] -> pure $ maybe (Left "could not parse as an email address") (Right . Just) $ toEmail x
           _ -> pure $ Left $ "Expected one value"
     , fieldView = \id name otherAttributes result isRequired ->