step_normalize() errors if only applied to one variable

[EmilHvitfeldt]

library(recipes)

recipe(~., data = mtcars) %>%
  step_normalize(disp) %>%
  prep()
#> Error in array(STATS, dims[perm]): 'dims' cannot be of length 0

recipe(~., data = mtcars) %>%
  step_normalize(disp, vs) %>%
  prep()
#> Recipe
#> 
#> Inputs:
#> 
#>       role #variables
#>  predictor         11
#> 
#> Training data contained 32 data points and no missing data.
#> 
#> Operations:
#> 
#> Centering and scaling for disp, vs [trained]

^{Created on 2022-04-22 by the reprex package (v2.0.1)}

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