Description:
------------
when provided a pattern that produces a zero-width match (empty string),
mb_ereg_search raises: `Warning: mb_ereg_search(): Empty regular expression`.

First the warning is misleading, since it should not warn about an 'empty regular expression', but about a 'zero-width match'.
More importantly this behavior is incorrect. A zero-width result is a perfectly valid match, and should never raise a warning.

For example, matching the pattern '\A' at position zero should succeed without a warning, and the resulting position should be zero.

Matching pattern '\s*' against the subject 'foo' at position 1 should succeed without a warning and the resulting position should be 1.

Test script:
---------------
<?php

$str = 'foo';
mb_ereg_search_init($str);
mb_ereg_search('\A');
var_dump(mb_ereg_search_getpos());
var_dump(mb_ereg_search_getregs());

mb_ereg_search('\s*');
var_dump(mb_ereg_search_getpos());
var_dump(mb_ereg_search_getregs());

mb_ereg_search('\w+');
var_dump(mb_ereg_search_getpos());
var_dump(mb_ereg_search_getregs());

mb_ereg_search('\Z');
var_dump(mb_ereg_search_getpos());
var_dump(mb_ereg_search_getregs());

Expected result:
----------------
int(0)
array(1) {
  [0]=>
  string(0) ""
}

int(0)
array(1) {
  [0]=>
  string(0) ""
}

int(3)
array(1) {
  [0]=>
  string(0) "foo"
}

int(3)
array(1) {
  [0]=>
  string(0) ""
}

Actual result:
--------------
Warning: mb_ereg_search(): Empty regular expression in ...
int(1)
array(1) {
  [0]=>
  string(0) ""
}

Warning: mb_ereg_search(): Empty regular expression in ...
int(2)
array(1) {
  [0]=>
  string(0) ""
}

int(3)
array(1) {
  [0]=>
  string(1) "o"
}

Warning: mb_ereg_search(): Empty regular expression in ...
int(4)
array(1) {
  [0]=>
  string(0) ""
}