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).
{
    return vMasterKey;
}
bool CWallet::HasEncryptionKeys() const
{
    return !mapMasterKeys.empty();
}
void CWallet::ConnectScriptPubKeyManNotifiers()
{
    for (const auto& spk_man : GetActiveScriptPubKeyMans()) {
        spk_man->NotifyWatchonlyChanged.connect(NotifyWatchonlyChanged);
        spk_man->NotifyCanGetAddressesChanged.connect(NotifyCanGetAddressesChanged);
    }
}
void CWallet::LoadDescriptorScriptPubKeyMan(uint256 id, WalletDescriptor& desc)
{
    auto spk_manager = std::unique_ptr<ScriptPubKeyMan>(new DescriptorScriptPubKeyMan(*this, desc));
    m_spk_managers[id] = std::move(spk_manager);
}
void CWallet::SetupDescriptorScriptPubKeyMans()