Handle the smoothstep degenerate case of empty range

[Malcolmnixon]

This PR fixes issue #68128 by handling the empty-range case as a binary-divisor between 0.0 and 1.0.
It also updates the documentation to describe positive and negative ranges.

Note that it depends on godotengine/godot-docs#9489 to go in first so the graph URL links work.

Bugsquad edit:

• Fix smoothstep returning out of range values for 0 length steps #68128

[Malcolmnixon]

@kleonc I copied your fix for the double implementation over to the float and squashed the commits in preparation for merge.

[kleonc]

LGTM!

But since I've proposed changing it to the current version, another opinion / approval would be good.

+Remainder for before merging:

Note that it depends on godotengine/godot-docs#9489 to go in first so the graph URL links work.

[Malcolmnixon]

I updated the PR to match the updated graph in the godotengine/godot-docs#9489 pull request. I would request people review that PR (which just adds the image) in order to get this dependent PR merged.

[Malcolmnixon]

The godot-docs image was converted from png to webp to align with the guidelines. As such this PR has been updated to refer to the webp image.

[clayjohn]

Looks good. Just needs a rebase before merging. Feel free to do that after 4.3 releases so you don't end up having to rebase again (since this needs to wait for 4.4 before merging)

[Malcolmnixon]

Looks good. Just needs a rebase before merging. Feel free to do that after 4.3 releases so you don't end up having to rebase again (since this needs to wait for 4.4 before merging)

Rebased

[akien-mga]

Thanks!