Nikkei ended the morning session almost flat Monday on a dearth of compelling trading cues and a low level of investor participation. The 225-issue Nikkei Stock Average was up 19.66 points, or 0.14 percent, from Friday to 13,669.77. The broader Topix index of all First Section issues on the Tokyo Stock Exchange was up 0.64 point, or 0.06 percent, at 1,143. 29. Gainers were led by oil, metal products and construction issues, while decliners included utilities, airlines and steelmakers.