[ruby/prism] 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 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

2 files changed, 24 insertions, 8 deletions

diff --git a/test/prism/fixtures/xstring.txt b/test/prism/fixtures/xstring.txt
index fcbaad91ae..7ec09468d8 100644
--- a/test/prism/fixtures/xstring.txt
+++ b/test/prism/fixtures/xstring.txt
@@ -7,3 +7,7 @@
 %x{
   foo
 }
+
+``
+
+%x{}
diff --git a/test/prism/snapshots/xstring.txt b/test/prism/snapshots/xstring.txt
index 56ba77a8c0..04b4cbf6ea 100644
--- a/test/prism/snapshots/xstring.txt
+++ b/test/prism/snapshots/xstring.txt
@@ -1,8 +1,8 @@
-@ ProgramNode (location: (1,0)-(9,1))
+@ ProgramNode (location: (1,0)-(13,4))
 ├── locals: []
 └── statements:
-    @ StatementsNode (location: (1,0)-(9,1))
-    └── body: (length: 4)
+    @ StatementsNode (location: (1,0)-(13,4))
+    └── body: (length: 6)
         ├── @ XStringNode (location: (1,0)-(1,7))
         │   ├── flags: ∅
         │   ├── opening_loc: (1,0)-(1,3) = "%x["
@@ -47,9 +47,21 @@
         │   ├── content_loc: (5,1)-(5,4) = "foo"
         │   ├── closing_loc: (5,4)-(5,5) = "`"
         │   └── unescaped: "foo"
-        └── @ XStringNode (location: (7,0)-(9,1))
+        ├── @ XStringNode (location: (7,0)-(9,1))
+        │   ├── flags: ∅
+        │   ├── opening_loc: (7,0)-(7,3) = "%x{"
+        │   ├── content_loc: (7,3)-(9,0) = "\n  foo\n"
+        │   ├── closing_loc: (9,0)-(9,1) = "}"
+        │   └── unescaped: "\n  foo\n"
+        ├── @ XStringNode (location: (11,0)-(11,2))
+        │   ├── flags: ∅
+        │   ├── opening_loc: (11,0)-(11,1) = "`"
+        │   ├── content_loc: (11,1)-(11,1) = ""
+        │   ├── closing_loc: (11,1)-(11,2) = "`"
+        │   └── unescaped: ""
+        └── @ XStringNode (location: (13,0)-(13,4))
             ├── flags: ∅
-            ├── opening_loc: (7,0)-(7,3) = "%x{"
-            ├── content_loc: (7,3)-(9,0) = "\n  foo\n"
-            ├── closing_loc: (9,0)-(9,1) = "}"
-            └── unescaped: "\n  foo\n"
+            ├── opening_loc: (13,0)-(13,3) = "%x{"
+            ├── content_loc: (13,3)-(13,3) = ""
+            ├── closing_loc: (13,3)-(13,4) = "}"
+            └── unescaped: ""