  "mempoolminfee": 0.00000000

When the minrelaytxfee is > 0.

{
    if (blockindex == nullptr)
    {
        if (chainActive.Tip() == nullptr)
            return 1.0;
        else
            blockindex = chainActive.Tip();
    }
    int nShift = (blockindex->nBits >> 24) & 0xff;
    double dDiff =
        (double)0x0000ffff / (double)(blockindex->nBits & 0x00ffffff);
    while (nShift < 29)
    {
        dDiff *= 256.0;
        nShift++;
    }
    while (nShift > 29)
    {
        dDiff /= 256.0;
        nShift--;
    }
    return dDiff;
}
UniValue blockheaderToJSON(const CBlockIndex* blockindex)