rules: run vol_id on opticals only if media is found

Opening an optical drive device node without O_NONBLOCK autocloses the
tray, we run vol_id on every media change by kernel emitted "change"
events, which can make it hard to change the media when the tray closes
immediatey again.:) We check for cdrom_id to indicate an existing track,
if no media is found, we will not open the device with vol_id.

Thanks to Christian Krause and DavidZ for debugging and testing.

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