When Nextcloud performs an upgrade or clean installation,
it will check whether /var/www/html/{config,data,custom_apps,themes} exist.
If not, it will copy
/usr/src/nextcloud/{config,data,custom_apps,themes} to /var/www/html.

This leads to a problem: If those subdirectories are existent but
empty, it will not do the copy. This situation is common when you mount
volumes to those subdirectories, like:

```
version: "2.1"
services:
    app:
        image: nextcloud:12-apache
        volumes:
            - nextcloud:/var/www/html:Z
            - nextcloud-custom_apps:/var/www/html/custom_apps:Z
            - nextcloud-config:/var/www/html/config:Z
            - nextcloud-data:/var/www/html/data:Z
            - nextcloud-themes:/var/www/html/themes:Z
        ports:
            - 8080:80/tcp
    db:
        image: mariadb
        volumes:
            - db:/var/lib/mysql:Z
        environment:
            MYSQL_USER: nextcloud
            MYSQL_DATABASE: nextcloud
            MYSQL_PASSWORD: nextcloud
            MYSQL_ROOT_PASSWORD: nextcloud

volumes:
    nextcloud:
    nextcloud-custom_apps:
    nextcloud-config:
    nextcloud-data:
    nextcloud-themes:
    db:
```

This patch will fix this issue by copying to those subdirectories when they
are empty.