[FIX] calendar: do not try to escape empty date
[FIX] calendar: do not try to escape empty date

display_start may be False and crash

storm-computers/odoo,storm-computers/odoo,hip-odoo/odoo,laslabs/odoo,bplancher/odoo,ygol/odoo,storm-computers/odoo,hip-odoo/odoo,hip-odoo/odoo,stephen144/odoo,sysadminmatmoz/OCB,bplancher/odoo,ygol/odoo,stephen144/odoo,microcom/odoo,storm-computers/odoo,dfang/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,ygol/odoo,stephen144/odoo,storm-computers/odoo,bplancher/odoo,sysadminmatmoz/OCB,laslabs/odoo,storm-computers/odoo,dfang/odoo,dfang/odoo,Elico-Corp/odoo_OCB,bplancher/odoo,ygol/odoo,dfang/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,microcom/odoo,hip-odoo/odoo,stephen144/odoo,bplancher/odoo,dfang/odoo,Elico-Corp/odoo_OCB,hip-odoo/odoo,stephen144/odoo,ygol/odoo,microcom/odoo,sysadminmatmoz/OCB,sysadminmatmoz/OCB,laslabs/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,ygol/odoo,microcom/odoo,Elico-Corp/odoo_OCB,sysadminmatmoz/OCB,hip-odoo/odoo,microcom/odoo,sysadminmatmoz/OCB,sysadminmatmoz/OCB,stephen144/odoo,ygol/odoo,microcom/odoo,dfang/odoo,laslabs/odoo