Silently casting an empty string, null or false into an object by adding a property

Silently casting an empty string, null or false into an object by adding a property
is pretty non-intuitive. If the same value was 1 or true you get a warning and it halts.

Since we can't break BC completely (yet) lets bump this from E_STRICT.

Also added a new section to UPGRADING for engine changes.

<?php
$x = '';
// $x = null;
// $x = false;
$x->baz = 1;
var_dump($x);

$y = 1;
$y->baz = 1;
var_dump($y);