Fix for spline PathOptimizer::getSleeve

Good day

I was playing around with this lib and stuck with error that on this line

if (t.containsPoint(this.poly.end)) {

t is undefined It's because method private getSleeve(sourceTriangle: Tr) gets sourceTriangle === undefined. Looking in code I found that this method used only ones

    const sleeve: FrontEdge[] = this.getSleeve(this.findSourceTriangle(localCdt))
    if (sleeve == null) {

so result of this method could be null or undefined and I decided to add check for sourceTriangle like

  private getSleeve(sourceTriangle: Tr): FrontEdge[] {
    if (!sourceTriangle) return;

and error gone

Why this.findSourceTriangle(localCdt) returns undefined I don't know, but I can give you my graph in json and code how I use it.

Start in PathOptimizer at getSleeve and trace the findSourceTriangle(localCdt) call that can provide an undefined sourceTriangle. Reproduce the failure with the graph JSON and usage code mentioned by the reporter, then verify that the spline path optimization handles the missing triangle without dereferencing an undefined value.