I'm sometimes getting an invalid json reply from `getwalletinfo` when rescanning is in progress.

The invalid part is this: `"scanning":{"duration":19,"progress":}}` (no value for `progress`).

I can't reliably reproduce this, but I did manage to run into this 3 times now (once every 10-20 attempts or so).

I'm running v0.20.0 and testing against a regtest instance. This happens while doing a batched `importmulti` of 2000 addresses (none of which have history).
{
    
    for (ScriptPubKeyMan* spk_man : GetAllScriptPubKeyMans()) {
        
        
        if (spk_man->SignTransaction(tx, coins, sighash, input_errors)) {
            return true;
        }
    }
    
    return false;
}
TransactionError CWallet::FillPSBT(PartiallySignedTransaction& psbtx, bool& complete, int sighash_type, bool sign, bool bip32derivs, size_t * n_signed) const
{
    if (n_signed) {
        *n_signed = 0;
    }
    LOCK(cs_wallet);
    
    for (unsigned int i = 0; i < psbtx.tx->vin.size(); ++i) {
        const CTxIn& txin = psbtx.tx->vin[i];
        PSBTInput& input = psbtx.inputs.at(i);
        if (PSBTInputSigned(input)) {
            continue;
        }