The option "level-seed" is not present in server.properties by default

The bug

In the server.properties file level-seed isn't there and isn't generating when turning on server.

Adding the line manually allows the server to generate a world with a custom seed.

How to reproduce

1. Start a server, and wait for the files to generate
2. Read through server.properties
   No line for the level seed exists

Expected behavior

The level-seed line would be present in the server.properties file by default, but would be left blank.