[Gecko Bug 1915286] Use 0rad as the default value if the path length is zero.

[moz-wptsync-bot]

When ths path length is zero, we may get (0, 0) as the tangent angle from ComputePointAtLength(). Using (0, 0) to atan2() may get an undefined result. We should avoid this, so use 0rad. This makes more sense and match other browsers.

Differential Revision: https://phabricator.services.mozilla.com/D220353

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1915286 gecko-commit: 6d8f66466c14f4e7b337a1ae61f6acf4807fe371 gecko-reviewers: dholbert