fix part abbreviations rendered when part names weren't rendered
fix part abbreviations rendered when part names weren't rendered

this was necessary after the EngravingRules refactor where the RenderPartNames setter was removed.
(fixing the implementation of #841)

ideally we'd provide an option to render part abbreviations but not part names,
but that would require extra settings,
and it doesn't seem too useful or common.

opensheetmusicdisplay/opensheetmusicdisplay,opensheetmusicdisplay/opensheetmusicdisplay,opensheetmusicdisplay/opensheetmusicdisplay