Description:
------------
While '' works as an array key, it does not work with ArrayObject. It should either work both or none (first one preferred).

Reproduce code:
---------------
php > $foo = new ArrayObject();
php > $foo[''] = 'lala';
PHP Fatal error:  Uncaught exception 'InvalidArgumentException' with message 'An offset must not begin with \0 or be empty' in php shell code:1
Stack trace:
#0 php shell code(1): unknown()
#1 {main}
  thrown in php shell code on line 1

InvalidArgumentException: An offset must not begin with \0 or be empty in php shell code on line 1

Call Stack:
   16.7554      63916   1. {main}() php shell code:0

Expected result:
----------------
Same as for arrays.

php > $foo = array();
php > $foo[''] = 'lala';
php > var_dump($foo);
array(1) {
  [""]=>
  string(4) "lala"
}


Actual result:
--------------
PHP Fatal error:  Uncaught exception 'InvalidArgumentException' with message 'An offset must not begin with \0 or be empty' in php shell code:1
Stack trace:
#0 php shell code(1): unknown()
#1 {main}
  thrown in php shell code on line 1

InvalidArgumentException: An offset must not begin with \0 or be empty in php shell code on line 1

Call Stack:
   16.7554      63916   1. {main}() php shell code:0