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 (m_internal_spk_managers.count(OutputType::LEGACY) == 0) {
        return false;
    }
    auto spk_man = dynamic_cast<LegacyScriptPubKeyMan*>(m_internal_spk_managers.at(OutputType::LEGACY));
    return spk_man != nullptr;
}
DescriptorScriptPubKeyMan* CWallet::GetDescriptorScriptPubKeyMan(const WalletDescriptor& desc) const
{
    for (auto& spk_man_pair : m_spk_managers) {
        
        DescriptorScriptPubKeyMan* spk_manager = dynamic_cast<DescriptorScriptPubKeyMan*>(spk_man_pair.second.get());
        if (spk_manager != nullptr && spk_manager->HasWalletDescriptor(desc)) {
            return spk_manager;
        }
    }
    return nullptr;
}
ScriptPubKeyMan* CWallet::AddWalletDescriptor(WalletDescriptor& desc, const FlatSigningProvider& signing_provider, const std::string& label, bool internal)
{
    if (!IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS)) {
        WalletLogPrintf("Cannot add WalletDescriptor to a non-descriptor wallet\n");