Fix scattergl animation index out of range in for loop

[BJohnBraddock]

Goal: Fix runtime error when "animating" scattergl traces. I don't believe scattergl traces actually support animations like easing and redraw=False, but they should at least not hit a runtime error.

As shown in #6251 , there is an unsafe index into cdata with variable i because variable i comes from scene.count and not cdata.length. This is normally fine. However, during frame redraw animations, we may call scattergl.plot with a subset of calcdata, so cdata.length can be < scene.count.

Further up in traces/scattergl/plot.py there was already an instance of checking if cdata[i] is undefined, so I've mirrored that into the for loop.

Fixes#6251.
This should also fix#6897 which is the same error as #6251.

[camdecoster]

Thanks for the PR! Before we review, could you please provide some videos showing the change?

[BJohnBraddock]

before.mp4

after.mp4

animate_scattergl.json

[camdecoster]

Thanks for the fix! It does take care of #6251 and #6897. I made a small syntax suggestion.

[BJohnBraddock]

Thanks for the review @camdecoster !