fix: do not quit web driver if not instantiated yet
fix: do not quit web driver if not instantiated yet

It can happen, that Karma is shut down before the driver is initialized.
jackTheRipper/karma-sauce-launcher,karma-runner/karma-sauce-launcher,jerkovicl/karma-sauce-launcher,matthewjh/karma-sauce-launcher,johanneswuerbach/karma-sauce-launcher,karma-runner/karma-sauce-launcher,jackspirou/karma-sauce-launcher