[ruby-core:99293] [Ruby master Bug#16519] pp [Hash.ruby2_keywords_hash({})] shows `[nil]`

Issue #16519 has been updated by nagachika (Tomoyuki Chikanaga).

Backport changed from 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: REQUIRED to 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: DONE

Seems already backported at bb93659fefd7f4557129043742771a33bd30c255.

----------------------------------------
Bug #16519: pp [Hash.ruby2_keywords_hash({})] shows `[nil]`
https://bugs.ruby-lang.org/issues/16519#change-86682

* Author: Eregon (Benoit Daloze)
* Status: Closed
* Priority: Normal
* Assignee: jeremyevans0 (Jeremy Evans)
* ruby -v: ruby 2.8.0dev (2020-01-21T13:45:10Z master 5798d35ff6) [x86_64-linux]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: DONE
----------------------------------------
This happens on `master`:

```
$ ruby -ve 'ruby2_keywords def flag(*a); a.last; end; pp [flag(**{})]'
ruby 2.8.0dev (2020-01-21T13:45:10Z master 5798d35ff6) [x86_64-linux]
[nil]
```

Of course it should be `[{}]`, as it is for `pp [{}]`.

On 2.7.0 it warns (should be fixed, it's valid to `pp` a flagged Hash):
```
$ ruby -ve 'ruby2_keywords def flag(*a); a.last; end; pp [flag(**{})]'
ruby 2.7.0p0 (2019-12-25 revision 647ee6f091) [x86_64-linux]
[/home/eregon/.rubies/ruby-2.7.0/lib/ruby/2.7.0/pp.rb:226: warning: Passing the keyword argument as the last hash parameter is deprecated
/home/eregon/.rubies/ruby-2.7.0/lib/ruby/2.7.0/pp.rb:334: warning: The called method is defined here
{}]
```

The warning being in the middle of the output is a fun fact here.
Lines it refers to (still the same on current master):
https://github.com/ruby/ruby/blob/v2_7_0/lib/pp.rb#L226
https://github.com/ruby/ruby/blob/v2_7_0/lib/pp.rb#L334

This is very confusing as it can happen during `test-all` and then show output such as:
```
<[{:a=>1}]> expected but was
<[{:a=>1}, nil]>.
```
when the reality is (can be verified with `p` before the `assert_equal`):
```
<[{:a=>1}]> expected but was
<[{:a=>1}, {}]>.
```



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