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).
{
    
    
    
    int start_height = 0;
    uint256 start_block;
    bool start = chain().findFirstBlockWithTimeAndHeight(startTime - TIMESTAMP_WINDOW, 0, FoundBlock().hash(start_block).height(start_height));
    WalletLogPrintf("%s: Rescanning last %i blocks\n", __func__, start ? WITH_LOCK(cs_wallet, return GetLastBlockHeight()) - start_height + 1 : 0);
    if (start) {
        
        ScanResult result = ScanForWalletTransactions(start_block, start_height, {} , reserver, update);