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 4f77d8d328. Most calls don't pass
keywords.

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

Benchmark results:

```
named_multi_arg_splat
after:    5344097.6 i/s
before:   3088134.0 i/s - 1.73x  slower

named_post_splat
after:    5401882.3 i/s
before:   2629321.8 i/s - 2.05x  slower

anon_arg_splat
after:   12242780.9 i/s
before:   6845413.2 i/s - 1.79x  slower

anon_arg_kw_splat
after:   11277398.7 i/s
before:   4329509.4 i/s - 2.60x  slower

anon_multi_arg_splat
after:    5132699.5 i/s
before:   3018103.7 i/s - 1.70x  slower

anon_post_splat
after:    5602915.1 i/s
before:   2645185.5 i/s - 2.12x  slower

anon_kw_splat
after:   15403727.3 i/s
before:   6249504.6 i/s - 2.46x  slower

anon_fw_to_named_splat
after:    2985715.3 i/s
before:   2049159.9 i/s - 1.46x  slower

anon_fw_to_named_no_splat
after:    2941030.4 i/s
before:   2100380.0 i/s - 1.40x  slower

fw_to_named_splat
after:    2801008.7 i/s
before:   2012416.4 i/s - 1.39x  slower

fw_to_named_no_splat
after:    2742670.4 i/s
before:   1957707.2 i/s - 1.40x  slower

fw_to_anon_to_named_splat
after:    2309246.6 i/s
before:   1375924.6 i/s - 1.68x  slower

fw_to_anon_to_named_no_splat
after:    2193227.6 i/s
before:   1351184.1 i/s - 1.62x  slower
```