Avoid JsonWriter multi-write error

JsonWriter requires that its clients pass it only a single top-level object.
The existing implementation tries to make code cleaner by having mJsonWriter
never be null, and instead use a global static "NULL_JSON_WRITER" that just
discards data.  But because JsonWriter complains if more than one top-level
object is passed, making this a global object does not work.  This change
instead copes with mJsonWriter being null.

Change-Id: Ia37ccfc8646e91f11a64713dd92d2846eb86ac54