President Joe Biden has faced small but persistent opposition in Democratic primaries so far, with about one in 10 Democratic voters choosing “uncommitted” in states where that’s an option, but MSNBC’s Claire McCaskill tried to soothe concerns about his strength heading into the general election. The average Biden protest vote, whether voters are choosing “uncommitted” or picking long-shot candidates, has been about 13 percent so far, but the former Missouri senator told “Morning Joe” that incumbent presidents often face opposition from fatigued voters in primaries who turn out for them in Nov…