[IMP] Dataset#get_id_index returns null instead of -1 when no record found
[IMP] Dataset#get_id_index returns null instead of -1 when no record found

In order to pair with convention on Dataset#index === null for creation mode

gisce/openerp-web,gisce/openerp-web,splbio/openerp-web,vnc-biz/openerp-web,splbio/openerp-web,MarkusTeufelberger/openerp-web,gisce/openerp-web,MarkusTeufelberger/openerp-web,vnc-biz/openerp-web,vnc-biz/openerp-web,MarkusTeufelberger/openerp-web,vnc-biz/openerp-web,splbio/openerp-web