Added a test for nil handling
Added a test for nil handling

Added this test to confirm the behaviour when nil is supplied as the fee
imput. In theory, nil fee should never be supplied, but it's good to
handle that case so that no error is unexpectedly thrown. Because if
nil fee is supplied then nil remission should be given.

ministryofjustice/fr-staffapp,ministryofjustice/fr-staffapp,ministryofjustice/fr-staffapp,ministryofjustice/fr-staffapp