ssd_hc_bcanz errors if single bootstrap sample

fix for 1 and ensure also possible with 0

``` r
library(ssdtools)

fits <- ssd_fit_bcanz(ssddata::ccme_boron)
ssd_hc_bcanz(fits, nboot = 1)
#> Error in `dplyr::group_by()` at ssdtools/R/hcp.R:150:5:
#> ! Must group by variables found in `.data`.
#> ✖ Column `value` is not found.
```

^{Created on 2025-06-05 with [reprex v2.1.1](https://reprex.tidyverse.org)}