Export custom XML exports content properties (os_propertyentry) for pages that are not included

When you export a space to XML and use the "Custom Export" option, then content properties of pages that are not exported (due to restrictions) will get exported.

When you import such an export file you will end up with entries in the os_propertyentry table that have no link to a corresponding page. These entries are unreachable from now on and will not get deleted when you delete the space.
Consequentially every further import of the space will fail with a duplicate key exception.

Import failed. Check your server logs for more information. com.atlassian.confluence.importexport.ImportExportException: Unable to complete import: could not insert: [bucket.user.propertyset.BucketPropertySetItem#BucketPropertySetItem[entityId=2129925,entityName=confluence_ContentEntityObject,key=com.k15t.scroll.versions.scroll-page-title,type=6,boolean=false,string=,text=Restricted,int=0,double=0.0,long=0,date=]]

Steps to reproduce

1. Create a space
2. As user A (with 'restrict pages' permission) create a new page and use a plugin that uses content properties to store stuff (e.g. create a task list)
3. As user A restrict view and edit to user A
4. As user B (e.g. spaceadmin) export the space to XML with "Custom Export"
5. You can find the content properties in the entities.xml, but no corresponding page object
6. Delete the space
7. Import the space export
8. Delete the space. You will still find the content properties in the os_propertyentry table.
9. Import the space again and get the error from above.