don't include null values in the query string

I'm attempting to use this assembly for my API and to pass (optional) oData query parameters.

For example:
```cs
Task<IList<TimesheetLineModel>> Get([AliasAs("id")] int businessId, [AliasAs("$orderby")] string orderBy = null, [AliasAs("$filter")] string filter = null);
```

In the case where I don't supply a value for those orderBy/filter parameters, they shouldn't be included in the query string.

At the moment, it just dies with a null reference exception.