EZP-31806: Casted destinationContentId to int

[ViniTou]

Question	Answer
JIRA issue	EZP-31806
Type	bug
Target eZ Platform version	v3.x
BC breaks	no
Tests pass	yes
Doc needed	no

As stated in \eZ\Publish\Core\FieldType\ImageAsset\Value

    /**
     * Related content id's.
     *
     * @var mixed|null
     */
    public $destinationContentId;

destination content ID could be also a string, as it is not utilized now and there is always int passed in terms of creating external connectors this needs to be unified in one way or another, till that it seems only fair to handle in both and cast that ID to int before passing to service.

Checklist:

• PR description is updated.
• Tests are implemented.
• Added code follows Coding Standards (use $ composer fix-cs).
• PR is ready for a review.