Fix buildGeometry with no faces

[davepagurek]

Resolves #8206

Changes:

• Checks for when tessellation produces no results, and in that case, leaves the input vertices alone
• Now that the default PATH shape mode might not always be turned into TRIANGLES via tessellation, I've updated the GeometryBuilder to handle that case and not create any faces (the vertices are only there to be used in edges)

Here's an updated version of the test case in the issue, which now draws a line instead of drawing nothing: https://editor.p5js.org/davepagurek/sketches/cCcrr7Ytj

PR Checklist

• npm run lint passes
• Inline reference is included / updated
• Unit tests are included / updated