Drawing by appending to the selected layer fails on empty layers

[Keavon]

You can use the Pen, Spline, or Freehand tool to draw while holding Shift to append to an existing selected layer of vector content. But if you create a brand new layer and draw on it with Shift held, this is the result:

Whereas this is the normal result of drawing (where a new layer appears):

So we need to detect if no existing vector data is there, and if so, add the Path and Stroke nodes instead of the Flatten Vector Elements node.

It also needs to work on "empty" layers that do contain modifier nodes, like this:

Currently, append-drawing results in this:

But the desired output in this case should be: