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).
{
    
    if (IsImmatureCoinBase())
        return 0;
    CAmount credit = 0;
    if (filter & ISMINE_SPENDABLE) {
        
        credit += GetCachableAmount(CREDIT, ISMINE_SPENDABLE);
    }
    if (filter & ISMINE_WATCH_ONLY) {
        credit += GetCachableAmount(CREDIT, ISMINE_WATCH_ONLY);
    }
    return credit;
}
CAmount CWalletTx::GetImmatureCredit(bool fUseCache) const
{
    if (IsImmatureCoinBase() && IsInMainChain()) {
        return GetCachableAmount(IMMATURE_CREDIT, ISMINE_SPENDABLE, !fUseCache);
    }
    return 0;
}
CAmount CWalletTx::GetAvailableCredit(bool fUseCache, const isminefilter& filter) const