[5.7] Fix BelongsToMany pivot relationship child with loaded relations wakeup

[erikgaal]

Overview

This PR demonstrates the bug that was reported in #23068. Although a previous PR (#23081) was submitted and merged to fix the issue, another edge case caused the bug to persist.

Reproduction

Whenever a model is serialized that has a loaded pivot relation that is using a Pivot class, and that pivot class has loaded relations, the following exception is thrown:

Illuminate\Database\Eloquent\RelationNotFoundException : Call to undefined relationship [pivot] on model [Illuminate\Tests\Integration\Queue\Role].

The bug persists because in the case that there are loaded relations within the pivot, they are still appended to the result of getQueueableRelations() as relation.pivot.child, even if the relation method pivot does not exist.

framework/src/Illuminate/Database/Eloquent/Model.php

Lines 1410 to 1426 in a0ba83d

	foreach ($this->getRelations() as $key => $relation) {
	if (method_exists($this, $key)) {
	$relations[] = $key;
	}
	if ($relation instanceof QueueableCollection) {
	foreach ($relation->getQueueableRelations() as $collectionValue) {
	$relations[] = $key.'.'.$collectionValue;
	}
	}
	if ($relation instanceof QueueableEntity) {
	foreach ($relation->getQueueableRelations() as $entityKey => $entityValue) {
	$relations[] = $key.'.'.$entityValue;
	}
	}
	}

Proposed Solution

In order to prevent any children from a pivot (or non-existing relationship) to be serialized, the foreach in getQueueableRelations() should continue when the relationship method does not exist.