Optional properties in a Gradle script

I have my uploadArchives configuration in a separate script, and it uses the properties “ossrhUsername” and “ossrhPassword”. See this for an idea of how my project is organized: https://github.com/simplyianm/bukkit-bootstrap/blob/287b83b5b0749068348cc50826de54fa23e22f85/bootstrap/bootstrap.gradle

When my friend imports the project into IntelliJ, he gets an error about ossrhUsername not being defined. How can I define ossrhUsername to have a default value? Thanks.