[5.8] Make updateExistingPivot() safe on non-existent pivot

[mpyw]

Fixes issue from [5.8] Fix many to many sync results with custom pivot model by themsaid · Pull Request #28416 · laravel/framework.

Currently, when we use updateExistingPivot() with a custom pivot class, the following code will be executed.

$updated = $this->getCurrentlyAttachedPivots()
    ->where($this->foreignPivotKey, $this->parent->{$this->parentKey})
    ->where($this->relatedPivotKey, $this->parseId($id))
    ->first()
    ->fill($attributes)
    ->isDirty();

However, it is unsafe; If we call updateExistingPivot() for non-existent record, it will unexpectedly cause FatalThrowableError.

Symfony\Component\Debug\Exception\FatalThrowableError: Call to a member function fill() on null
  File "/var/www/vendor/laravel/framework/src/Illuminate/Database/Eloquent/Relations/Concerns/InteractsWithPivotTable.php", line xxx

This PR resolves this issue by fixing like this:

$pivot = $this->getCurrentlyAttachedPivots()
    ->where($this->foreignPivotKey, $this->parent->{$this->parentKey})
    ->where($this->relatedPivotKey, $this->parseId($id))
    ->first();

$updated = $pivot ? $pivot->fill($attributes)->isDitry() : false;