[5.5] Allow associate on MorphTo to accept null value

[segadora]

This will make the MorphTo match the signature of BelongsTo.
BelongsTo can accept a null value on associate.
MorphTo would fail due that getKey() would not exists on null.
This should be possible also due to that is it possible to generate migrations with nullable polymorphic relations

I would prefer if this also could be added to 5.4.

Example:

$model->user()->associate($user);
$model->customer()->associate($customer);
if ($morph_relation) {
    $model->morphRelation()->associate($morph_relation);
}

Can be turned into

$model->user()->associate($user);
$model->customer()->associate($customer);
$model->morphRelation()->associate($morph_relation);

[taylorotwell]

Merged.