@Override
    public boolean hasReservedCapacity(@Nonnull UUID txnId) {
        if (txnId.equals(NULL_UUID)) {
            return false;
        }
        return reservedCapacityCountByTxId.containsKey(txnId);
    }
@Test
    public void null_uuid_has_no_reserved_capacity() {
        assertFalse(counter.hasReservedCapacity(TxnReservedCapacityCounter.NULL_UUID));
    }