fix: add default database name

[saw-jan]

env VIRTUAL_HOST is used to determine the database name, and if VIRTUAL_HOST is not set while running the container with SQL=mysql and NEXTCLOUD_AUTOINSTALL=YES, the nextcloud installation fails because the --database-name= option has empty value

command:

# start mysql server
# start nextcloud
docker run --rm \
-e SERVER_BRANCH=stable32 \
-e NEXTCLOUD_AUTOINSTALL=true \
-e SQL=mysql \
ghcr.io/juliusknorr/nextcloud-dev-php84:master

⌛ Waiting for other containers
 - MySQL
✅ Database server ready
⚠ No value for PHP_XDEBUG_MODE was found. Not updating the setting.
Nextcloud is not installed - only a limited number of commands are available
Add the host IP as host.docker.internal to /etc/hosts ...
database name will be 
🔧 Starting auto installation
occ maintenance:install --admin-user=admin --admin-pass=admin --database=mysql --database-name= --database-host=database-mysql --database-user=root --database-pass=nextcloud
   Enter the database name for MySQL/MariaDB
Nextcloud is not installed - only a limited number of commands are available
Last nextcloud.log entry:

=======================================================================================
🚨 Server installation failed.
=======================================================================================

[saw-jan]

CC @juliusknorr We need this fix so that we can use the master tag in the CI. See https://github.com/nextcloud/integration_openproject/actions/runs/21585684969/job/62193131621