This simple PR updates the logic underlying the gridsync.systray.Menu.populate method to only add the "Browse Documentation" and "Report Issue" help submenu items in the event that docs_url and issues_url are defined in the [help] section of config.txt. Since Gridsync does define these values, this doesn't change anything from a current user-facing perspective, however, it allows custom/whitelabel builds of Gridsync to more easily omit one or both actions in the future.