CRUD methods that take filter arrays should ensure that they are cast to objects to guarantee serialization to a BSON document. Server 3.0+ is more strict about types, and BSON arrays may no longer be allowed where objects are expected.

Note that this is inconsistent for some commands (e.g. count was relaxed in SERVER-15456), but we should be consistent throughout.

For methods where filters are truly optional, we should simply omit the value rather than casting an empty array.