Add support for :accept_nil on ForeignKeyLookupTransform

[thbar]

In some cases getting a nil value here is reasonable.

Original work:

• sgrgic@4695fac
• sgrgic@310ae99

Acceptance

• tests
• by default, nil should not be allowed
• if accept_nil is passed as true, a nil result is allowed
• if accept_nil is passed as false, a nil result is not allowed and an exception is thrown