Stock prices showed no clear trend Monday in the quietest trading of the year as traders looked ahead to a potentially volatile week. Takeover news and speculation and a late flurry of buying in blue chips provided the highlights of an otherwise uninspired session. The Dow Jones average of 30 industrials gained 15.09 to 2,050.07. But declining issues slightly outnumbered advances on the New York Stock Exchange, with 737 up, 794 down and 445 unchanged. Big Board volume came to 131.89 million shares, down from 200.02 million in the previous session, and the lightest total since a 111.58 million-share day Dec. 29. Analy were reluctant to take big positions in advance of the government's report Thursday on the nation's trade balance for January. The trade deficit narrowed in the last two months of 1987, but some analysts expect the sts said tradersJanuary data to show a modest increase in the gap between exports and imports. This week's trade report will come just ahead of a quarterly ``triple witching hour'' involving a group of expiring options and futures on stock indexes. The past few witching hours have been relatively uneventful with the adoption of new procedures designed to spread out the impact of last-minute maneuvering by computer program traders. Nevertheless, analysts say uncertainty aboe index closed at 296.94, up .85.