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_spk_managers.count(id) > 0) {
        return m_spk_managers.at(id).get();
    }
    return nullptr;
}
std::unique_ptr<SigningProvider> CWallet::GetSolvingProvider(const CScript& script) const
{
    SignatureData sigdata;
    return GetSolvingProvider(script, sigdata);
}
std::unique_ptr<SigningProvider> CWallet::GetSolvingProvider(const CScript& script, SignatureData& sigdata) const
{
    for (const auto& spk_man_pair : m_spk_managers) {
        if (spk_man_pair.second->CanProvide(script, sigdata)) {
            return spk_man_pair.second->GetSolvingProvider(script);
        }
    }
    return nullptr;
}
LegacyScriptPubKeyMan* CWallet::GetLegacyScriptPubKeyMan() const