the way we use PSBTs to sign things requires that we have the
scriptpubkey available on the utxo so we can populate the witness-utxo
field with it.
this causes problems if we don't already have the scriptpubkey cached in
the database, as in *some* cases we require a round trip to the HSM to
populate them
to get over this hump, we backfill any and all missing scriptpubkey
information for the utxo's that we hold in our wallet.
this will allow us to clean up the NULL handling of missing
scriptpubkeys.
niftynei