A suicide bomber killed herself Monday when she exploded a vehicle packed with explosives near a stadium where Nigeria’s president had just held an election rally in the northeastern city of Gombe, the police said. There were no other casualties. The explosion occurred in a nearly empty area about a mile from the stadium, which President Goodluck Jonathan had just left, Deputy Superintendent Fwaje Atajiri said. No one immediately claimed responsibility. Most suicide bombings in Nigeria are attributed to Boko Haram extremists, who have vowed to disrupt the Feb. 14 elections for president, state governors and legislators.