OSX: Fix QGeoPositionInfo attribute values with corelocation plug-in

Fix corelocation plugin to return "nan" values for the QGeoPositionInfo
attributes which aren't available.

Task-number: QTBUG-38300
Change-Id: Id4c6eb69ef65fccfd7de32ec476d3afa3c4bbf98
Reviewed-by: Aaron McCarthy <mccarthy.aaron@gmail.com>
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>