If the new CLI enable-plugin option is invoked with the -restart option, it will restart Jenkins even if no plugins are enabled. If no plugins are enabled as the result of this command then there should not be a restart.
To reproduce, disable the gradle plugin in the UI. Then the first iteration of this command: java -jar jenkins-cli.jar -s http://localhost:8080/ -auth user:token enable-plugin gradle -restart. will report:
Enabling plugin `gradle' (1.28)
Plugin `gradle' was enabled.
If you execute the command again, there will be no output as the plugin was already enabled. However, Jenkins will still be restarted.