[8.x] Check for incomplete class to prevent unexpected error when class cannot be loaded

[the-real-christoph]

Since PHP 7.2 is_object returns true even when class could not be correctly instantiated.

Currently trying to retry jobs of unknown classes (e.g. the job was created by another system) is crashing with "method_exists(): The script tried to execute a method or access a property of an incomplete object."

The change is aiming to allow job retry functionality independent from availability of class in the current context.

[GrahamCampbell]

Thanks for the PR. Can you add a test here please?

[taylorotwell]

Curious why you wrapped the last two conditions in parens?

[the-real-christoph]

Curious why you wrapped the last two conditions in parens?

Mainly readability, otherwise would look like "!$instance instanceof __PHP_Incomplete_Class"

Test will be added soon. @GrahamCampbell