Allow explicit specification of strata = NULL for grouped resampling

[hfrick]

closes #485

library(rsample)

set.seed(11)
rs1 <- group_vfold_cv(warpbreaks, "tension")

rs1 <- group_vfold_cv(warpbreaks, "tension", strata = NULL)

^{Created on 2024-05-22 with reprex v2.1.0}

[simonpcouch]

This does the trick!

Is there a reason why this change is in check_grouped_strata() rather than just changing the condition in:

rsample/R/vfold.R

Lines 220 to 222 in c82c2a2

	if (!missing(strata)) {
	strata <- check_grouped_strata({{ group }}, {{ strata }}, pool, data)
	}

to if (!is.null(strata))?

[hfrick]

yeah... just doing that switch breaks being able to supply an unquote name for strata. I think it would need to be evaluated for this condition but unevaluated for check_grouped_strata() and that looked like the worse effort/effect ratio than what I did there.

I have since noticed that I was not alone in going down this route: #100

If we're touching this, we should do it across rsample.

[github-actions]

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