BUG- translating "!= null" in list options builder

Given a list call like this:

```c#
client.Product.ListAsync(opts => opts.AddFilter(p => p.ShipFromAddressID != null);
```

The resulting API call looks like this:

```
/products?ShipFromAddressID=!
```

"Not null" should translate to `*`, so we actually want this:

```
/products?ShipFromAddressID=*
```
