ConfigEntityBase::__sleep() serializes plugin instances if they were not previously initialized

Problem/Motivation

The method tries to do two things:

a) It ensures that values for initialized plugin objects in plugin collections are stored
b) Preventing those plugin collections from being serialized by removing them from $vars.

The problem is that b) doesn't work if the plugin collection object was not previously initialized already.

I've seen this happening with filter config entities, which caches the filter config entities (is this really necessary?) immediately after loading them.

That means when __sleep() runs $vars = get_object_vars($this);, $vars['pluginCollection'] is NULL. After that we call getPluginCollections() which initializes it, but in $vars, the property is still null. Then we compare the object against the values in $vars, it's not matching anything and we're not removing it.

The extra sad part is that the whole thing is unnecessary since the object wasn't instantiated in the first place.

Proposed resolution

As a quickfix, first get the plugin collections and after that the $vars.

A nicer fix would be to run this logic *only* on initialized plugin collections, and completely skip it for this case. I guess this needs a method/interface that extends ObjectWithPluginCollectionInterface, or maybe this is entity specific.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet