Should full backup content xml file be empty or not added at all to include all?

Fast Solution:

AndroidManifest.xml

<application
    android:allowBackup="true"
    android:fullBackupContent="true"
    ...
    ...
    ...
</application>

For more details see: https://developer.android.com/guide/topics/data/autobackup

If you want to silence the warnings, but don't need to exclude any files from the backup, you could also just add tools:ignore="AllowBackup" to the application tag in your AndroidManifest.xml to suppress the warning (or use the Suppress button that you get when viewing the warning details).

you can add android:fullBackupContent="true" in your androidmanifest.xml