When setting the checkbox "Envronment Variables" in global configuration without adding list of variables - `buildEnvironment` in `Computer` and many other actions starts throwing null pointer exception.

I think the problem is in here:

// Computer.java
for (NodeProperty nodeProperty: Jenkins.getInstance().getGlobalNodeProperties()) {
nodeProperty.buildEnvVars(env,listener);
}

// EnvironmentVariablesNodeProperty.java
@Override 
public void buildEnvVars(EnvVars env, TaskListener listener) throws IOException, InterruptedException {
     env.putAll(envVars);
} 

getGlobalProperties returns NodeProperty with `envVars` that is null.

I think we should either remove the checkbox on save when no variable was added or think of a better UI for it. 

Also in server side we should add protection against this issue as this completely blocked all my builds until I figured out what was the problem