[Prism] Use `putnil` for nil kwargs, not `putobject {}`

This addresses one of the issues in the `test_kw_splat_nil` failure, but
doesn't make the test pass because of other changes that need to be made
to Prism directly.

One issue was when we have the following code Prism was using
`putobject` with an empty hash whereas the parse.y parser used `putnil`.

```ruby
:ok.itself(**nil)
```

Before:

```
0000 putobject                              :ok                       (   1)[Li]
0002 putobject                              {}
0004 opt_send_without_block                 <calldata!mid:itself, argc:1, KW_SPLAT>
0006 leave
```

After:

```
== disasm: #<ISeq:<main>@test2.rb:1 (1,0)-(1,17)>
0000 putobject                              :ok                       (   1)[Li]
0002 putnil
0003 opt_send_without_block                 <calldata!mid:itself, argc:1, KW_SPLAT>
0005 leave
```

Related to ruby/prism#2935.

0 files changed, 0 insertions, 0 deletions