Fix Session::get('key') returns null when value is (int) 0

[dafriend]

Issue #2334

Description
The code in question is from Session.php starting at line #499

if (! empty($key) && $value = dot_array_search($key, $_SESSION))
{
    return $value;
}

When dot_array_search() returns a value of 0 (zero) then $value is evaluated as false so $value is not returned eventhough it should be.

Proposed fix: change the conditional to

if (! empty($key) && ! is_null($value = dot_array_search($key, $_SESSION)))

A unit test added to confirm this works.