* gnus-sum.el (gnus-update-marks): Do not remove empty 'unexist'
  ranges, since `nnimap-retrieve-group-data-early' also uses it as a flag
  to see whether the group was synced before.

  See also:

  http://thread.gmane.org/gmane.emacs.gnus.general/82121
  http://thread.gmane.org/gmane.emacs.gnus.general/83231