This PR fixes an AST and token incompatibility between Parser gem and `Prism::Translation::Parser`
for empty xstring literal.

## Parser gem (Expected)

```console
$ bundle exec ruby -Ilib -rparser/ruby33 -ve \
'buf = Parser::Source::Buffer.new("/tmp/s.rb"); buf.source = "``"; p Parser::Ruby33.new.tokenize(buf)'
ruby 3.3.0 (2023-12-25 revision https://github.com/ruby/prism/commit/5124f9ac75) [x86_64-darwin22]
[s(:xstr), [], [[:tXSTRING_BEG, ["`", #<Parser::Source::Range /tmp/s.rb 0...1>]],
[:tSTRING_END, ["`", #<Parser::Source::Range /tmp/s.rb 1...2>]]]]
```

## `Prism::Translation::Parser` (Actual)

Previously, the AST and tokens returned by the Parser gem were different:

```console
$ bunele exec ruby -Ilib -rprism -rprism/translation/parser33 -ve \
'buf = Parser::Source::Buffer.new("/tmp/s.rb"); buf.source = "``"; p Prism::Translation::Parser33.new.tokenize(buf)'
ruby 3.3.0 (2023-12-25 revision https://github.com/ruby/prism/commit/5124f9ac75) [x86_64-darwin22]
[s(:xstr, s(:str, "")), [], [[:tBACK_REF2, ["`", #<Parser::Source::Range /tmp/s.rb 0...1>]],
[:tSTRING_END, ["`", #<Parser::Source::Range /tmp/s.rb 1...2>]]]]
```

After this correction, the AST and tokens returned by the Parser gem are the same:

```console
$ bundle exec ruby -Ilib -rprism -rprism/translation/parser33 -ve \
'buf = Parser::Source::Buffer.new("/tmp/s.rb"); buf.source = "``"; p Prism::Translation::Parser33.new.tokenize(buf)'
ruby 3.3.0 (2023-12-25 revision https://github.com/ruby/prism/commit/5124f9ac75) [x86_64-darwin22]
[s(:xstr), [], [[:tXSTRING_BEG, ["`", #<Parser::Source::Range /tmp/s.rb 0...1>]],
[:tSTRING_END, ["`", #<Parser::Source::Range /tmp/s.rb 1...2>]]]]
```

https://github.com/ruby/prism/commit/4ac89dcbb5