Add failing test: hasMany and null foreign key

This relates to https://github.com/laravel/framework/issues/5649

If Foo hasMany Bar, and a Bar exists with its foo_id set to null (i.e.
it does not belong to any Foo), Laravel will currently find that Bar
when looking for Bars related to a fresh unsaved Foo (which still has
an id of null), since the id and foreign key match. Null should never
be a foreign key match.

    class Foo extends Eloquent {
        public function bars() {
            return $this->hasMany('Bar');
        }
    }
    class Bar extends Eloquent {}
    $bar = Bar::create(); // Saved but no foo_id
    $foo = new Foo(); // Unsaved, still has id of null
    echo count($foo->bars); // Gives 1 (the $bar above), should give 0

Disclaimer: I am not certain I have written the test correctly.