fix: return canonical short prefix from Comment::prefix

`Comment::prefix` iterated `CommentKind::BY_PREFIX` forward, so `/**/`
matched the `/**/` entry itself and returned the full 4-char string.
`block_to_line` then computed `&text[4..text.len()-2]` = `&text[4..2]`
and panicked.

Delegate `Comment::prefix` to `CommentKind::prefix`, which already
iterates in reverse and returns the canonical short form (`/*` for
non-doc block comments). The `/**/` and `/***` entries in `BY_PREFIX`
are still needed for `from_text` to classify them as non-doc blocks.

fixes #22071

Author: Albab-Hasan

crates/ide-assists/src/handlers/convert_comment_block.rs

@@ -381,6 +381,21 @@ fn main() {
         );
     }
 
+    #[test]
+    fn empty_block_to_line() {
+        check_assist(
+            convert_comment_block,
+            r#"
+/**/$0
+fn main() {}
+"#,
+            r#"
+
+fn main() {}
+"#,
+        );
+    }
+
     #[test]
     fn end_of_line_block_to_line() {
         check_assist_not_applicable(

crates/syntax/src/ast/token_ext.rs

@@ -32,11 +32,7 @@ impl ast::Comment {
     }
 
     pub fn prefix(&self) -> &'static str {
-        let &(prefix, _kind) = CommentKind::BY_PREFIX
-            .iter()
-            .find(|&(prefix, kind)| self.kind() == *kind && self.text().starts_with(prefix))
-            .unwrap();
-        prefix
+        self.kind().prefix()
     }
 
     /// Returns the textual content of a doc comment node as a single string with prefix and suffix