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).
{
    std::set<ScriptPubKeyMan*> spk_mans;
    for (bool internal : {false, true}) {
        for (OutputType t : OUTPUT_TYPES) {
            auto spk_man = GetScriptPubKeyMan(t, internal);
            if (spk_man) {
                spk_mans.insert(spk_man);
            }
        }
    }
    return spk_mans;
}
std::set<ScriptPubKeyMan*> CWallet::GetAllScriptPubKeyMans() const
{
    std::set<ScriptPubKeyMan*> spk_mans;
    for (const auto& spk_man_pair : m_spk_managers) {
        spk_mans.insert(spk_man_pair.second.get());
    }
    return spk_mans;
}
ScriptPubKeyMan* CWallet::GetScriptPubKeyMan(const OutputType& type, bool internal) const