RSpec 3.2: How to test render inside a private method of controller?

I'm new to rspec and I'm using RSpec 3.2. I have a method in my controller which is like,

class MainController < ApplicationController
  before_filter :get_items
  //other methods
 def get_items
   if @user.admin?
     @user_items = [ModuleName].fetchItems
   end

   if @user_items.empty?
     flash.now[:error] = "No items for the user"
     render "index"
   end
 end
end

And my RSpec looks like this,

require "rails_helper"

 RSpec.describe MainController, :type => :controller do

  describe '#get_items' do
     it 'gets the items for admin' do
      @controller = MainController.new
      @controller.instance_eval{ get_items }   # invoke the private method
      expect(@controller.instance_eval{ @user_items }).not_to be_blank
      expect(response).not_to render_template("index")
      end
  end
end

The above rspec works fine when I keep @user.admin? as true. How do I test the other 'IF' block where it renders the index and also mock the value of '@user.admin?' to be true or false? Please note that '@user.admin?' comes from another module in my lib folder.