Support empty parameters

[SamHurne]

[Moved from Codeplex: https://gw2dotnet.codeplex.com/workitem/1342]

The API will soon support empty query parameters. This means that we don't have to check for that case anymore, and just let the API deal with it.

See: https://github.com/arenanet/api-cdi/issues/24

TODO (up for grabs): remove all code that checks for empty parameters

NOTE: the code should still assert that collection types are not null. For collections, the only change is that a collection argument with Count == 0 is now a valid argument.