Fix: with no `SERVER_PASSWORD` can still apply setting

[win5923]

Choices

• Allow setting ServerPassword to an empty string through env SERVER_PASSWORD
• Resolve the issue of applying settings without manually modifying PalWorldSettings.ini when there is no server password.
• Addressing a similar issue as described in https://github.com/thijsvanloef/palworld-server-docker/issues/275#issue-2115930445.

Test instructions

1. Comment out the SERVER_PASSWORD or set it to an empty string.
2. Open the game and check if the ServerPassword in PalWorldSettings.ini is set to an empty string.

Checklist before requesting a review

• [x] I have performed a self-review of my code
• [x] I've added documentation about this change to the README.
• [x] I've not introduced breaking changes.

[win5923]

Do I need to apply this setting to ADMIN_PASSWORD as well?

[Dashboy1998]

Do I need to apply this setting to ADMIN_PASSWORD as well?

rcon-cli doesn't work with an empty password. RCON protocol itself refuses all requests when no password is set.

If there is no admin password then you aren't able to become an admin: /adminpassword ""

[thijsvanloef]

@win5923 Will review tonight!