[5.x] Fix structure not being saved to collection

[jasonvarga]

Fixes #13014

The issue is that when Collection::handleExists() on the line above gets called, it pulls the collections from the cache, which gets the dirty state attributes, which includes getting the structure, which blinks it. The one in the cache doesn't have a structure, so null gets stored in blink.

Then, when the actual collection is saved in a moment, checks the blink for the structure, needs the null that was put there, and writes nothing.

This started happening since #11150 because now that the Collection class uses the HasDirtyState trait, these kick in:

cms/src/Stache/Stores/BasicStore.php

Lines 27 to 29 in fa108ea

	if (method_exists($item, 'syncOriginal')) {
	$item->syncOriginal();
	}

I don't love this fix but it'll do for now until there's more time for something better.