BUGFIX: Skip null when publishing nodes

[kdambekalns]

The getNodeFromContextPath(…) in the Neos.Ui NodeService may return null, passing that to publishNodde(…) fails:

Exception in line 240 of /…/Flow_Object_Classes/Neos_Neos_Ui_Controller_BackendServiceController.php:
Neos\Neos\Service\PublishingService_Original::publishNode(): Argument #1 ($node) must be of type
Neos\ContentRepository\Domain\Model\NodeInterface, null given, called in
/…/Neos_Neos_Ui_Controller_BackendServiceController.php on line 240

This change adds a check and skips those nodes.

How to verify it

I see this error in a client project quite often, but sadly do not have a reproducer. Looking at the code shows this can happen, though.

[kdambekalns]

I found this on my machine today (after quite some time trying to find a difference in installed packages between local and production setups). PhpStorm's local history says the code was added on October 16th, but I don't remember writing it. 🙃 So maybe it's from somewhere else, but I could not find anything. So I created this PR.

[skurfuerst]

makes sense, thanks :)