Simplify uses of defined?

If an instance variable isn't defined it will always return nil.

Author: jdufresne

app/helpers/rails_admin/application_helper.rb

@@ -122,7 +122,7 @@ def navigation(parent_groups, nodes, level = 0)
         nav_icon = node.navigation_icon ? %(<i class="#{node.navigation_icon}"></i>).html_safe : ''
         css_classes = ['nav-link']
         css_classes.push("nav-level-#{level}") if level > 0
-        css_classes.push('active') if defined?(@action) && current_action?(@action, model_param)
+        css_classes.push('active') if @action && current_action?(@action, model_param)
         li = content_tag :li, data: {model: model_param} do
           link_to nav_icon + " " + node.label_plural, url, class: css_classes.join(' ')
         end

lib/generators/rails_admin/templates/rails_admin.scss.erb

@@ -1 +1 @@
-<%= instance_variable_defined?(:@fa_font_path) ? %{$fa-font-path: "#{@fa_font_path}";\n} : '' %>@import "rails_admin/src/rails_admin/styles/base";
+<%= @fa_font_path ? %{$fa-font-path: "#{@fa_font_path}";\n} : '' %>@import "rails_admin/src/rails_admin/styles/base";

lib/rails_admin/config/model.rb

@@ -48,9 +48,7 @@ def initialize(entity)
       end
 
       def excluded?
-        return @excluded if defined?(@excluded)
-
-        @excluded = !RailsAdmin::AbstractModel.all.collect(&:model_name).include?(abstract_model.try(:model_name))
+        @excluded ||= !RailsAdmin::AbstractModel.all.collect(&:model_name).include?(abstract_model.try(:model_name))
       end
 
       def object_label