By Michelle Nichols UNITED NATIONS (Reuters) – China accused the United States of double standards at the United Nations on Friday for challenging Beijing’s sovereignty over Taiwan while emphasizing the principle of sovereignty for Ukraine after Russian forces invaded. A day after Chinese leader Xi Jinping warned U.S. President Joe Biden in a phone call against playing with fire over Taiwan, deputy U.N. Ambassador Geng Shuang reinforced the message during a meeting on Ukraine at the 15-member U.N. Security Council. “While some country has repeatedly emphasized the principle of sovereignty over…