Fix specs, add issue number to TODOs comments

ee/app/controllers/groups/epics_controller.rb

@@ -68,7 +68,7 @@ class Groups::EpicsController < Groups::ApplicationController
   end
 
   # we need to override the default state which is opened for now because we don't have
-  # states for epics and need all as default for navigation to work correctly
+  # states for epics and need all as default for navigation to work correctly (#4017)
   def set_default_state
     params[:state] = 'all'
   end

ee/app/finders/epics_finder.rb

@@ -4,7 +4,8 @@ class EpicsFinder < IssuableFinder
   end
 
   def execute
-    raise ArgumentError, 'group_id argument is missing' unless group
+    raise ArgumentError, 'group_id argument is missing' unless params[:group_id]
+    return [] unless group
 
     items = init_collection
     items = by_created_at(items)
@@ -19,7 +20,7 @@ class EpicsFinder < IssuableFinder
     execute.count
   end
 
-  # we don't have states for epics for now this method
+  # we don't have states for epics for now this method (#4017)
   def count_by_state
     {
       all: row_count

ee/app/policies/ee/group_policy.rb

@@ -31,6 +31,8 @@ module EE
       end
 
       rule { auditor }.enable :read_group
+      rule { admin }.enable :read_epic
+      rule { has_projects }.enable :read_epic
 
       rule { admin | (can_owners_manage_ldap & owner) }.enable :admin_ldap_group_links