name = NULL argument gives surprising results with enframe()

``` r
tibble::enframe(1:3, name = NULL)
#> # A tibble: 3 x 2
#>   value  `NA`
#>   <int> <int>
#> 1     1     1
#> 2     2     2
#> 3     3     3
```

Created on 2018-08-05 by the [reprex package](http://reprex.tidyverse.org) (v0.2.0).