Allow for clamping the zero vector when possible

[oddbookworm]

I was playing around with clamp_magnitude and I noticed that pygame.Vector2().clamp_magnitude(1) would always result in a ValueError being raised, even though a length of 0 is a perfectly fine output here. I've just modified it to follow these examples:

pygame.Vector2().clamp_magnitude(5) # should be allowed
pygame.Vector2().clamp_magnitude(0, 5) # should be allowed
pygame.Vector2().clamp_magnitude(0.1, 5) # should not be allowed

The first two should be fine to return the zero vector, the last one is impossible to do because it's ambiguous in what direction it should go in (just like normalizing the zero vector is impossible)

[ankith26]

This seems like a reasonable change of behaviour, thanks for the PR 🎉

[MyreMylar]

LGTM 👍