Let `complete()` behave better when nesting `NULL`

[psychelzh]

When passing NULL to nesting() in calling complete(), it behaves differently than when not passing nesting() to complete(). From user's aspect, it is more intuitive if they should behave similarly.

library(tidyr)
df <- data.frame(x = factor("A", LETTERS[1:2]))
complete(df, x)
#> # A tibble: 2 x 1
#>   x    
#>   <fct>
#> 1 A    
#> 2 B
complete(df, x, nesting(NULL))
#> # A tibble: 1 x 1
#>   x    
#>   <fct>
#> 1 A

^{Created on 2021-12-09 by the reprex package (v2.0.1)}