least and greatest should ignore nulls

[Fil]

d3.least([1, null]) returns null, and d3.greatest([-1, null]) returns null, contrary to the other reducers (such as min, max, median…) which are all skipping nulls.

Interestingly, d3.leastIndex and d3.greatestIndex do not return the same result when using the default and identity accessors (because it follows two different code paths):

d3.leastIndex([1, null]); // 1 [bug]
d3.leastIndex([1, null], d => d); // 0 [correct answer]

This issue is similar to #203.