Grouped resampling breaks with non-missing `strata = NULL`

[hfrick]

The default for strata is NULL but specifying it explicitly breaks check_grouped_strata().

library(rsample)

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

rs1 <- group_vfold_cv(warpbreaks, "tension", strata = NULL)
#> Error in .subset2(x, i, exact = exact): attempt to select less than one element in get1index

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

[github-actions]

This issue has been automatically locked. If you believe you have found a related problem, please file a new issue (with a reprex: https://reprex.tidyverse.org) and link to this issue.