fix _last_exit_status(): don't break if exit status is not an integer

rc.xsh

@@ -7,7 +7,7 @@ xontrib load vox z
 def _last_exit_status():
     try:
         exit_status = __xonsh_history__.rtns[-1]
-        return exit_status if exit_status > 0 else None
+        return exit_status if exit_status != 0 else None
     except IndexError:
         return None
 $PROMPT_FIELDS['last_exit_status'] = _last_exit_status