It might be null. We wrap the retrieval of the data stream version in
QmlDebugClient and return the minimum if connection is null. The extra
copy of the connection QmlEngine is dropped as QmlDebugClient already
has one and we don't want to hit a dangling pointer.

Change-Id: Ida8c45d357d46b4942eea99b77065d3c51c7edb9
Reviewed-by: hjk <hjk@qt.io>