Description:
------------
I'm using version 2.0.0 of php-yaml.

According to http://www.yaml.org/spec/current.html#id2527268, empty streams are valid, they don't have to include a document.
Take the example provided in the spec, where the stream only consists of comments.

Test script:
---------------
yaml_parse('');

yaml_parse('#Only comment');

Expected result:
----------------
array(0) {
}

or

string(0) ""

or even

bool(true)

Actual result:
--------------
PHP Warning:  yaml_parse(): end of stream reached without finding document 0

bool(false)