Things are looking up on Fifth Avenue’s upper-Midtown shopping corridor. First, a large, long-awaited retail lease has been signed at 680 Fifth Ave. at East 54th Street — and sources said it’s with Pop Mart, makers of creepy but oddly lovable Labubu dolls. Negotiations for the 7,000 square-foot space were first reported by The Post’s Lois Weiss last October but the deal wasn’t yet done. We earlier reported a similar-sized Pop Mart lease at 1540 Broadway in Times Square. Cushman & Wakefield’s