Fix error when saving nested non-persistent resource

[timothyqiu]

Found this bug when tweaking the resource format saver.

If a property is PROPERTY_USAGE_RESOURCE_NOT_PERSISTENT and is a resource containing another new resource, saving the scene / resource will fail:

Resource was not pre cached for the resource section, bug?

In the engine, only ImageTexture* has PROPERTY_USAGE_RESOURCE_NOT_PERSISTENT properties. And those properties don't have nested resource, thus won't trigger this bug.

Fixes #92820

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To reproduce this bug, use this MRP (test-4.zip).

1. Open new_resource.tres in the inspector.
2. Click "Generate" property (it won't change, just to set temporary value for the actual non-persistent property)
3. Click save.