Fix beamChain right-angles

[LazerPlatypus]

Fix findPoint function to handle is90or270 properly by checking for undef rather than 0

Existing behavior does not handle right-angles in beamChain correctly.
This was fixed previously, but there may have been regression with a change of OpenSCAD treating '0' as falsy before, and not now, However, I cannot confirm this.

The code snippit below used to cause the following error:
WARNING: cos() parameter could not be converted: argument 0: expected number, found string ("error") in file <Round-Anything/polyround.scad>, line 687

use <Round-Anything/polyround.scad>;
let (
    beam_radii_points = beamChain(
        radiiPoints=[[0, 0, 0], [40, 0, 0], [40, 40, 0], [0, 40, 0]],
        offset1=0.5,
        offset2=-0.5,
        startAngle=90,
        endAngle=90,
        mode=0,
    )
) {
    polyRoundExtrude(
        radiiPoints= beam_radii_points,
        length=5,
        r1=0,
        r2=0,
        fn=20,
        convexity=30,
    );
}

The above code now (correctly) creates the following model: