Description:
------------
In some cases the preg_split strips non-empty pieces when the PREG_SPLIT_NO_EMPTY flag i set. Specifically when using assertions only to split on, eg. the string to split on itself is actually empty (but not the pieces)

Reproduce code:
---------------
$terms = preg_split('/(?<=\d)(?=[a-z???])/', 'ser1ia456l', 0, PREG_SPLIT_NO_EMPTY);
print_r($terms);


Expected result:
----------------
Array
(
    [0] => ser1
    [1] => ia456
    [2] => l
)

Actual result:
--------------
Array
(
    [0] => ser1
    [1] => ia456
)