fix #31760, regression in Dict iterate

[JeffBezanson]

This uses 0 as the sentinel instead of nothing. Hopefully somebody can comment on whether this preserves the benefit of the original change.

[JeffBezanson]

@nanosoldier runbenchmarks("collection" || "iteration", vs = ":master")

[vchuravy]

Should be fine since the original change was about not doing typemax(Int)+1 iirc

[nanosoldier]

Your benchmark job has completed - possible performance regressions were detected. A full report can be found here. cc @ararslan

[ndinsmore]

@JeffBezanson & @KristofferC would it be prudent to dig in and understand why this fix works, or more specifically why the original version was slow? I ask because the original version was based around similar semantics to iterate and the slowdown you fixed here could be all over the place with implementations of iterate.

[KristofferC]

I think the problem here was basically that there was a nested union with Nothing which doesn't seem to be handled well by the union splitter. Or something like that. I don't think it is all over the place.

[JeffBezanson]

Yes, I think that's it. iterate uses a union of Nothing and a tuple, while this had a Union{Nothing,Int} inside the tuple as well.