[ruby-core:107900] [Ruby master Bug#18633] proc { |a, **kw| a } autosplats and treats empty kwargs specially

Issue #18633 has been reported by Eregon (Benoit Daloze).

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Bug #18633: proc { |a, **kw| a } autosplats and treats empty kwargs specially
https://bugs.ruby-lang.org/issues/18633

* Author: Eregon (Benoit Daloze)
* Status: Open
* Priority: Normal
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
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```ruby
irb(main):005:0> proc { |a| a }.call([1, 2])
=> [1, 2]
irb(main):006:0> proc { |a, **kw| a }.call([1, 2])
=> 1 # should be [1, 2]
irb(main):007:0> proc { |a, kw: 42| a }.call([1, 2])
=> 1 # should be [1, 2]
```

What's the reason for `proc { |a, **kw| a }` to autosplat?
It seems inconsistent with the resolution of #16166, and it seems nobody would want that behavior (it loses arguments but the user extremely likely did not want that).
Could we change it so procs never autosplat, just like `proc { |a| a }`.

My understanding of the change in #16166 is to reflect the fact positional and kwargs are separated, and so adding `**kw` or `kw:` should never change anything if only positional arguments are passed.
This breaks in this case though.

Also I noticed:
```ruby
irb(main):010:0> proc { |a, **kw| a }.call([1, 2])
=> 1
irb(main):011:0> proc { |a, **kw| a }.call([1, 2], **{})
=> [1, 2]
```
Which is really unfortunate as it shows a difference between passing `**{}` or nothing.
AFAIK passing `**{}` or nothing should always be equivalent, but it breaks in this case.

(from https://bugs.ruby-lang.org/issues/16166#note-14)



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