- Nuke empty_string. It is a reminanent from the time where RETURN_FALSE()

- Nuke empty_string. It is a reminanent from the time where RETURN_FALSE()
  used to return "" and not bool(false). It's not worth keeping it because
  STR_FREE() and zval_dtor() always have to check for it and it slows down
  the general case. In addition, it seems that empty_string has been abused
  quite a lot, and was used not only for setting zval's but generally in
  PHP code instead of "", which wasn't the intention. Last but not least,
  nuking empty_string should improve stability as I doubt every place
  correctly checked if they are not mistakenly erealloc()'ing it or
  calling efree() on it.
  NOTE: Some code is probably broken. Each extension maintainer should
  check and see that my changes are OK. Also, I haven't had time to touch
  PECL yet. Will try and do it tomorrow.