kendalltau function now returns a nan tuple if empty arrays used as para...

...meters instead of raising a runtime exception. Previously, empty arrays would have cause a maximum recursion depth exceeded error in mergesort if the provided arrays were empty (issue #3884).
`assert_equal` has code to handle `nan`, so this can be simplified to `assert_equal(stats.kendalltau([], []), (np.nan, np.nan))`.