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 (!crypter.Encrypt(_vMasterKey, kMasterKey.vchCryptedKey))
        return false;
    {
        LOCK(cs_wallet);
        mapMasterKeys[++nMasterKeyMaxID] = kMasterKey;
        WalletBatch* encrypted_batch = new WalletBatch(*database);
        if (!encrypted_batch->TxnBegin()) {
            delete encrypted_batch;
            encrypted_batch = nullptr;
            return false;
        }
        encrypted_batch->WriteMasterKey(nMasterKeyMaxID, kMasterKey);
        for (const auto& spk_man_pair : m_spk_managers) {
            auto spk_man = spk_man_pair.second.get();
            if (!spk_man->Encrypt(_vMasterKey, encrypted_batch)) {
                encrypted_batch->TxnAbort();
                delete encrypted_batch;
                encrypted_batch = nullptr;
                
                
                assert(false);
            }
        }
        
        SetMinVersion(FEATURE_WALLETCRYPT, encrypted_batch, true);
        if (!encrypted_batch->TxnCommit()) {
            delete encrypted_batch;
            encrypted_batch = nullptr;