kolab-util-calendar: fixups in kolab_util_calendar_store()
kolab-util-calendar: fixups in kolab_util_calendar_store()

* default timezone argument
  - may be NULL (don't assert otherwise)
  - is unused (explicitly ignore it)
* turn PFB trigger failure into warning
  (this is not fatal to object storing)

GNOME/evolution-kolab,GNOME/evolution-kolab