[11.x] Fix add and remove plan

[driesvints]

Fixes #922

[Nuranto]

@driesvints I don't know the code, I just saw your commit, so maybe I will say something stupid.
But should'nt we create the single plan item if not exist before setting plan to null ?

Something like this :

if ($this->hasSinglePlan()) {
           if ($this->items()->count() == 0 ) { // Can occurs after upgrading from version 10 to 11
                   $this->items()->create([
                           'stripe_id' => $subscription->stripe_id,
                           'stripe_plan' => $subscription->stripe_plan,
                           'quantity' => $subscription->quantity,
                   ]);
            }
            $this->fill([
                'stripe_plan' => null,
                'quantity' => null,
            ])->save();
}

If you think that's ugly, I think https://github.com/laravel/cashier/blob/master/UPGRADE.md should be updating to explicitly announce that the items should be created manually.

[driesvints]

@Nuranto no that's not needed. Your webhooks will take care of that.