Fix an AST and token incompatibility for Prism::Translation::Parser

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 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 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 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>]]]]
```

Author: koic

lib/prism/translation/parser/compiler.rb

@@ -1678,7 +1678,7 @@ def visit_x_string_node(node)
             children, closing = visit_heredoc(node.to_interpolated)
             builder.xstring_compose(token(node.opening_loc), children, closing)
           else
-            parts = if node.unescaped.lines.count <= 1
+            parts = if node.unescaped.lines.one?
               [builder.string_internal([node.unescaped, srange(node.content_loc)])]
             else
               start_offset = node.content_loc.start_offset

lib/prism/translation/parser/lexer.rb

@@ -326,7 +326,7 @@ def to_a
                 type = :tIDENTIFIER
               end
             when :tXSTRING_BEG
-              if (next_token = lexed[index][0]) && next_token.type != :STRING_CONTENT
+              if (next_token = lexed[index][0]) && next_token.type != :STRING_CONTENT && next_token.type != :STRING_END
                 type = :tBACK_REF2
               end
             end

test/prism/fixtures/xstring.txt

@@ -7,3 +7,7 @@
 %x{
   foo
 }
+
+``
+
+%x{}