PVector angleBetween returns 0 when x and y of either parameter are both 0

I found an existing report describing similar behavior with 3D PVectors (https://github.com/processing/processing/issues/2045), but it appears the same problem affects 2D PVectors as well.

When either the start or end PVector is (0,0) the output of angleBetween() is always 0.

I'm not particularly experienced with Processing, so it's entirely possible that I'm misunderstanding something or using the API incorrectly. I tend to pick it up every few months for small projects and then promptly forget most of what I learned.

That said, I've spent some time testing this behavior and looking into the implementation, and based on what I found, I would not expect the angle to be 0 in the cases I tested.

Edit: I forgot to add, I'm using processing 4.5.2

从 PVector.angleBetween() 入口开始，将 2D 行为与 issue #2045 中相关的 3D 报告进行比较。明确当任一向量为 (0,0) 时的预期结果，然后通过针对这些情况的回归覆盖验证修正后的行为。