In some edge cases, like when splitting a path on or very close to its
start or end point, `Path.split()` will return an array in which one of
the elements is not a Path object, but rather an empty array.
That is rather combersome to check for, so this changes that behaviour
and will return null for such cases.
I've also updated the documentation to clarify this behaviour.
Fixes#6816
This also fixes a bug when a path was being split on its end point.
eded9e9d3a