Function S2Loop.empty

A special vertex chain of length 1 that creates an empty loop (i.e., a loop with no edges that contains no points). Example usage:

static s2.util.math.vector.Vector!(double,3L)[] empty();

S2Loop emptyLoop = new S2Loop(S2Loop.empty());

The loop may be safely encoded lossily (e.g. by snapping it to an S2Cell center) as long as its position does not move by 90 degrees or more.