Use 0rad as the default value if the path length is zero.
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