[motion-path] Offset-rotate and zero length paths #971
Description
The Motion Paths Level 1 offset-rotate should define what is the directional tangent vector of a zero length path. Google Chrome seems to use value 0 in those cases, as shown by this example: https://jsfiddle.net/v4jo88mh/1/
The spec could also have a note saying that zero length subpaths are ignored when calculating the distance or rotation. Here's an example of a zero length subpath: https://jsfiddle.net/wnc1fht4/