Support Null As-Is in OBJECT_MARKER

[vjkoskela]

The OBJECT_MARKER allows the serialized form of any object to replace the entire value of the data block. For example, if you provided a String "ABC" then the serialized form would be:

{ ... "context":{ ... }, "data":"ABC" ... }

Similarly, the object could be a collection or complex class. Regardless of what the type is, the value may be null. The Steno format does not permit a null value for the "data" key. However, we should support this an option.

Specifically, if "allowNullData" (for example) is set to true then the value of "data" is allowed to be null. For example:

{ ... "context":{ ... }, "data":null ... }

The option should be set to false by default because it is strictly not compliant with the Steno specification but can be useful in some cases.