Explicitly disallow data frames with NA or "" names

[DavisVaughan]

Closes #6758

[otoomet]

I run into a confusing error:

> df <- data.frame(a = 1:2, b = 2:1)
> names(df)[1] <- ""
> mutate(df, x = 3)
Error in initialize(...) : attempt to use zero-length variable name
> packageVersion("dplyr")
[1] ‘1.1.0’

I understand that it is fixed now in a sense that mutate (& friends) will provide a more informative error message? I thought that something is wrong with my provided variable names ("x" in the example above).

Thanks for your work!

[DavisVaughan]

Looks like this in the dev version

df <- data.frame(a = 1:2, b = 2:1)
names(df)[1] <- ""
dplyr::mutate(df, x = 3)
#> Error in `dplyr::mutate()`:
#> ! Can't transform a data frame with `NA` or `""` names.