Thanks to the new semantics from [ruby-core:115808], `**nil` is now
equivalent to `**{}`. Since the only thing one could do with anonymous
keyword rest parameter is to delegate it with `**`, nil is just as good
as an empty hash. Using nil avoids allocating an empty hash.

This is particularly important for `...` methods since they now use
`**kwrest` under the hood after 4f77d8d3. Most calls don't pass
keywords.

    Comparison:
                             fw_no_kw
                    post:   9816800.9 i/s
                     pre:   8570297.0 i/s - 1.15x  slower