Updating a contribution to completed with a new trxn_id writes unjoinable finanical_trxn record

Steps to reproduce:

1. Create a new contribution in state "pending" with no transaction id (nominally via API or a BAO)
2. Attempt to complete the contribution via API Contribution.create(id => *, trxn_id => *, contribution_status_id => [Completed status id], ...)

Expected outcome:

a. Contribution status and transaction id is updated.
b. A financial transaction record is written with the relevant trxn_id

Actual outcome:

As expected but the trxn_id is NULL for part b.

This results in missing transactions data on reports as the civicrm_financial_trxn record can't be joined into, eg. civicrm_contribution