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);
    
    CKey seed_key;
    seed_key.MakeNewKey(true);
    CPubKey seed = seed_key.GetPubKey();
    assert(seed_key.VerifyPubKey(seed));
    
    CExtKey master_key;
    master_key.SetSeed(seed_key.begin(), seed_key.size());
    for (bool internal : {false, true}) {
        for (OutputType t : OUTPUT_TYPES) {
            auto spk_manager = std::unique_ptr<DescriptorScriptPubKeyMan>(new DescriptorScriptPubKeyMan(*this, internal));
            if (IsCrypted()) {
                if (IsLocked()) {
                    throw std::runtime_error(std::string(__func__) + ": Wallet is locked, cannot setup new descriptors");
                }
                if (!spk_manager->CheckDecryptionKey(vMasterKey) && !spk_manager->Encrypt(vMasterKey, nullptr)) {
                    throw std::runtime_error(std::string(__func__) + ": Could not encrypt new descriptors");
                }
            }
            spk_manager->SetupDescriptorGeneration(master_key, t);
            uint256 id = spk_manager->GetID();
            m_spk_managers[id] = std::move(spk_manager);
            AddActiveScriptPubKeyMan(id, t, internal);
        }
    }
}
void CWallet::AddActiveScriptPubKeyMan(uint256 id, OutputType type, bool internal)