Fix bulk insert nulls into geometry not null column
Fix bulk insert nulls into geometry not null column

refs #2281

[groonga-dev,02095]

Reported by yoku0825. Thanks!!!

yoku0825/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,mroonga/mroonga,naoa/mroonga,naoa/mroonga,mroonga/mroonga,mroonga/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,naoa/mroonga,mroonga/mroonga,yoku0825/mroonga,yoku0825/mroonga