8022: Uncaught Error: Call to a member function addItem() on array in app/code/Magento/Sales/Model/Order/Shipment.php(backport to 2.2)

[RomaKis]

Description

Backport from #11680.
Method getTracksCollection() was able to return different types of data: collection or array. Now it will always return tracks collection object.

Fixed Issues (if relevant)

1. Uncaught Error: Call to a member function addItem() on array in app/code/Magento/Sales/Model/Order/Shipment.php #8022: Uncaught Error: Call to a member function addItem() on array in app/code/Magento/Sales/Model/Order/Shipment.php

Manual testing scenarios

1. Load or create a Shipment model

/*** @var $shipment \Magento\Sales\Model\Order\Shipment **/
$shipmentCollection = $this->_objectManager->get(
    'Magento\Sales\Model\ResourceModel\Order\Shipment\CollectionFactory'
)->create();
$shipment = $shipmentCollection->getFirstItem();
{code}
		
2. Make a call
`$shipment->getTracks();`
		
3. Create a Track model and try to add it to the Shipment

{code}
/*** @var $track \Magento\Sales\Model\Order\Shipment\Track **/
$track = $this->_objectManager->get(
    'Magento\Sales\Model\Order\Shipment\TrackFactory'
)->create();
$track->setNumber('Test Number')
          ->setTitle('Test Title')
          ->setCarrierCode('Test CODE');

$shipment->addTrack($track)->save();

Expected result

1. Successfull save

Actual result

You will receive an error Call to a member function addItem() on array because the method getTracksCollection() in Shipment model will return an array instead of Collection Object.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)