fix: empty variables are allowed

parser/src/lexer.rs

@@ -574,14 +574,8 @@ impl<'a> Lexer<'a> {
 
         match tok {
             b'?' => {
-                let start_digit = self.offset;
                 self.eat_while(|b| b.is_some() && b.unwrap().is_ascii_digit());
 
-                // empty variable name
-                if start_digit == self.offset {
-                    return Err(Error::BadVariableName((start, self.offset - start).into()));
-                }
-
                 Ok(Token {
                     value: &self.input[start + 1..self.offset], // do not include '? in the value
                     token_type: Some(TokenType::TK_VARIABLE),