By Niket Nishant and Sukriti Gupta (Reuters) -U.S. stock index futures were lower on Tuesday, as investors awaited a crucial labor market report, while fretting over likely delays in the release of key economic data due to a potential government shutdown. Investor sentiment has shifted back toward risk aversion, just a day after equities posted gains. While previous shutdowns have had limited impact on markets, some analysts warned that the current episode could be more disruptive given the delicate econ ...