fix unhelpful error message with '@' custom completion (#8620)

# Description

with such a line 
```nu
extern err [--flag: string@]
```
this pr changes the error message from
```nu
Error: nu::parser::unknown_command

  × Unknown command.
   ╭─[entry #69:1:1]
 1 │ extern err [--flag: string@]
   ·                            ▲
   ·                            ╰── unknown command
   ╰────
```
to
```nu
Error: nu::parser::parse_mismatch

  × Parse mismatch during operation.
   ╭─[entry #69:1:1]
 1 │ extern err [--flag: string@]
   ·                            ▲
   ·                            ╰── expected command name
   ╰────
```

crates/nu-parser/src/parser.rs

@@ -3082,27 +3082,25 @@ pub fn parse_shape_name(
         b"var-with-opt-type" => SyntaxShape::VarWithOptType,
         _ => {
             if bytes.contains(&b'@') {
-                let str = String::from_utf8_lossy(bytes);
+                let split: Vec<_> = bytes.split(|b| b == &b'@').collect();
-                let split: Vec<_> = str.split('@').collect();
+
-                let (shape, err) = parse_shape_name(
+                let shape_span = Span::new(span.start, span.start + split[0].len());
-                    working_set,
+                let cmd_span = Span::new(span.start + split[0].len() + 1, span.end);
-                    split[0].as_bytes(),
+                let (shape, err) = parse_shape_name(working_set, split[0], shape_span);
-                    Span::new(span.start, span.start + split[0].len()),
+
-                );
+                let command_name = trim_quotes(split[1]);
-                let command_name = trim_quotes(split[1].as_bytes());
+
+                if command_name.is_empty() {
+                    let err = ParseError::Expected("a command name".into(), cmd_span);
+                    return (SyntaxShape::Any, Some(err));
+                }
 
                 let decl_id = working_set.find_decl(command_name, &Type::Any);
 
                 if let Some(decl_id) = decl_id {
                     return (SyntaxShape::Custom(Box::new(shape), decl_id), err);
                 } else {
-                    return (
+                    return (shape, Some(ParseError::UnknownCommand(cmd_span)));
-                        shape,
-                        Some(ParseError::UnknownCommand(Span::new(
-                            span.start + split[0].len() + 1,
-                            span.end,
-                        ))),
-                    );
                 }
             } else {
                 return (SyntaxShape::Any, Some(ParseError::UnknownType(span)));