By Nupur Anand NEW YORK (Reuters) -CEOs of investment banking giants expressed optimism about a resurgence in capital markets when they reported fourth-quarter earnings on Tuesday on an improving deal pipeline and U.S. economy, but also warned of risks that could disrupt a nascent recovery. At Goldman Sachs, equities trading revenue jumped 26% in the fourth quarter versus a year earlier, sending shares up more than 1% in morning trading. Later in the day, the stock gave up its early gains and was trading flat. By contrast, trading revenue at rival Morgan Stanley was broadly flat, but investmen…