Racc 1.7.1 has been released:
https://github.com/ruby/racc/releases/tag/v1.7.1

This PR resolves rubocop/rubocop#12198.

```console
$ ruby-parse -e 'if foo..; end'
Failed on: (fragment:0)
/Users/koic/.rbenv/versions/3.3.0-dev/lib/ruby/gems/3.3.0+0/gems/parser-3.2.2.3/lib/parser/builders/default.rb:1676:
in `check_condition': undefined method `type' for nil (NoMethodError)

      case cond.type
               ^^^^^
        from /Users/koic/.rbenv/versions/3.3.0-dev/lib/ruby/gems/3.3.0+0/gems/parser-3.2.2.3/lib/parser/builders/default.rb:1707:
        in `check_condition'
        from /Users/koic/.rbenv/versions/3.3.0-dev/lib/ruby/gems/3.3.0+0/gems/parser-3.2.2.3/lib/parser/builders/default.rb:1275:
        in `condition'
```

I'm not sure if there's any significance to using the flip-flop syntax with `nil`,
but by using beginless or endless ranges and explicitly specifying `nil`,
the following current behaviors are made compatible:

## `iflipflop` with explicitly `nil`

```console
$ ruby-parse -e 'if foo..nil ; end'
(if
  (iflipflop
    (send nil :foo)
    (nil)) nil nil)
```

```console
$ ruby-parse -e 'if nil..bar ; end'
(if
  (iflipflop
    (nil)
    (send nil :bar)) nil nil)
```

## `eflipflop` with explicitly `nil`

```console
$ ruby-parse -e 'if foo...nil ; end'
(if
  (eflipflop
    (send nil :foo)
    (nil)) nil nil)
```

```console
$ ruby-parse -e 'if nil...bar ; end'
(if
  (eflipflop
    (nil)
    (send nil :bar)) nil nil)
```

The difference in the flip-flop with beginless or endless ranges is
that `s(:nil)` is replaced by `nil` in the flip-flop ASTs.

This is reflected in the tests.