Prohibit null RequestHandler

The SiriusFactory object should require that a non-null RequestHandler be passed to the createInstance method. That would catch the not-uncommon mistake of passing a null handler, which subsequently causes hard to debug errors.

Start at the SiriusFactory.createInstance entry point and trace how the RequestHandler argument is currently handled. Add coverage showing that a null handler is rejected, then run the relevant test suite and confirm that non-null handlers still create instances successfully.

Written by the indexing model from the issue text.