[fix] Auto-install script: fixed disabling OpenVPN #490

[Adityashandilya555]

When user selects 'n' to disable OpenVPN during auto-install, the script was setting VPN_DOMAIN to "example.com" instead of an empty string. This caused the OpenVPN container to still start and fail on systems without /dev/net/tun device.

Checklist

• [x] I have read the OpenWISP Contributing Guidelines.
• [x] I have manually tested the changes proposed in this pull request.
• [ ] I have written new test cases for new code and/or updated existing tests for changes to existing code.
• [ ] I have updated the documentation.

Reference to Existing Issue

Closes #490

Please open a new issue if there isn't an existing issue yet.

Description of Changes

The existing logic in init_command.sh already handles this case by exiting when VPN_DOMAIN is empty. This change ensures the variable is properly set to empty when OpenVPN is disabled.

Screenshot

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