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).
{
    AssertLockHeld(cs_wallet);
    unsigned int count = 0;
    for (auto spk_man : GetActiveScriptPubKeyMans()) {
        count += spk_man->KeypoolCountExternalKeys();
    }
    return count;
}
unsigned int CWallet::GetKeyPoolSize() const
{
    AssertLockHeld(cs_wallet);
    unsigned int count = 0;
    for (auto spk_man : GetActiveScriptPubKeyMans()) {
        count += spk_man->GetKeyPoolSize();
    }
    return count;
}
bool CWallet::TopUpKeyPool(unsigned int kpSize)
{
    LOCK(cs_wallet);
    bool res = true;
    for (auto spk_man : GetActiveScriptPubKeyMans()) {
        res &= spk_man->TopUp(kpSize);
    }
    return res;
}
bool CWallet::GetNewDestination(const OutputType type, const std::string label, CTxDestination& dest, std::string& error)