Letters should always have a reference, because that’s what DVLA use to
tell us when they’ve sent a letter.
If a letter has a reference of `None` then DVLA say they’ve sent a
letter with a reference of `'None'`. This means we can never reconcile
the letter, which means it stays in `created`, which means it never
gets billed.
We don’t think this has affected any real letters yet, just ones that
we’ve sent as tests.
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