September could go out with a bang for investors. - MarketWatch photo illustration/iStockphoto Good news on the economy this week should be good for stocks over the long run. But any economic news at all might be welcome. U.S. equities have performed well in September, despite it typically being a tough month for stocks. Yet a government shutdown now looks likely to hit midweek, right as investors will be closing out the month and quarter. Importantly, a shutdown could interrupt the release of September’ ...