Combat simulator does not work if only provide attacker hero and defenses

[Benjamin-Loison]

Adding an attacker troop solves the issue, if it is not intended.

For feature requests, you can delete this whole txt.

For bug reports, please use the folowing structure:

1. expected behaviour
2. incorrect behaviour
3. the operating system & version
4. PHP version on the server
5. MySQL / MariaDB version on the server