Wondering who’s driving this year’s advance in stocks? Individual investors have played a role, and so have companies themselves. But to date one notable class of professional speculators has been absent. It’s hedge funds, who have stubbornly refused to embrace stocks even as global equities added $10 trillion in value over the last three months. At the end of March, their net exposure as measured by the ratio of bullish bets to bearish ones stood near the lowest level in more than a year, client data compiled by JPMorgan’s prime brokerage unit showed.