There should always be a default constant for browser, platform, and environment, so if they are not set anywhere and the user runs a test class or suite without them, the test will run. Usually that is set to the default development environment platform.

Example:

public class Global_VARS {
    public static final String BROWSER = "firefox";
    public static final String PLATFORM = "Windows 10";
    public static final String ENVIRONMENT = "local";
    public static String DEF_BROWSER = null;
    public static String DEF_PLATFORM = null;
    public static String DEF_ENVIRONMENT = null;
}