`name` needs a default value in `use_rcpp()` ...

[jennybc]

name needs a default value in use_rcpp() or we should check name earlier and exit early and informatively if not provided.

Introduced in https://github.com/r-lib/usethis/commit/3c03f5cdeb6c96f528d019ea58931e8352f47dd7.

use_rcpp() now errors if called without name and the error is pretty confusing:

Error in `use_rcpp()`:
! Open file must be a code or test file.

It's actually coming from the internal helper compute_name(). This (inherited) documented behaviour:

#' @param name Either a string giving a file name (without directory) or
#'   `NULL` to take the name from the currently open file in RStudio.

doesn't make any sense for use_rcpp(), i.e. what does the currently open file have to do with it. And, as I just found, there might not even be a currently open file.

Check if this affects other functions, e.g. use_c(), that were also touched.