The generated Manifest still contains 'org.qtproject.example' as package name and displays this in 'General' Tab. Simply removing the package name gives a warning 'The Package Name is not valid'. From QT_ANDROID_PACKAGE_NAME docs I know, that the property must be removed completely, so I removed 

package="org.qtproject.example" 

from 'XML Source' Tab. Now I could compile the project without a package name in Manifest
Because I added nothing to CMakeLists.txt yet, Qt uses as default package 'org.qtproject.example.apptest_manifest' - sounds good.

Generating Templates from Qt 6.9.1 should generate a Manifest file without the package property and 'General' Tab should display 'Not Set' similar to Min and Target SDK which are also removed from Manifest since some time.
There's another problem: When package name property was removed and you switch between 'General' and 'XML Source' Tab, by magic the Manifest Editor added

package=""

under the hood.

workaround: before saving changes always go to 'XML Source' Tab, remove 

package=""

then save.