sql: to_hex differs from postgres with negative inputs

[maddyblue]

postgres=# select to_hex(-1);
  to_hex  
----------
 ffffffff

root@:26257/postgres> select to_hex(-1);
  to_hex  
+--------+
  -1      

[lsahn-gh]

Although @solongordon is the main assignee, I just gave it a try for the first PR because it has been about 8 days since it opened, but @solongordon looks busy. If you guys think that it is kinda rudely behavior, I am really sorry for that.
A PR would be on the board sooner.

Thanks!