This was changed by mistake in 23fafe98e3b4f4e15cdf764e1346d813d1eb8039: if
it's null we turn it into 0 (which is what the default call does, but it
does log BROKEN about it!):
```
2023-08-03T14:10:49.001Z **BROKEN** lightningd: Accessing a null column total_msat/15 in query SELECT id, status, destination, msatoshi, payment_hash, timestamp, payment_preimage, path_secrets, route_nodes, route_channels, msatoshi_sent, description, bolt11, paydescription, failonionreply, total_msat, partid, local_invreq_id, groupid, completed_at FROM payments ORDER BY id;
```
Fixes: #6501
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
881d7efd6e