By Michelle Nichols UNITED NATIONS (Reuters) -The United Nations on Monday said no ships involved in a U.N.-brokered Ukraine grain export deal were transiting a Black Sea maritime humanitarian corridor on the night of Oct. 29, when Russia says its vessels in Crimea were attacked. Russia has accused Ukraine of using air and maritime drones to target vessels in the Bay of Sevastopol early on Saturday and suggested one of the drones may have been launched from a civilian vessel chartered to export food from Ukrainian ports. Ukraine has neither confirmed nor denied it was behind the attack but say…