Correct validate epic_color_highlight FF in Epic mutations

The validation for the Epic `color` attribute was attempting to check
the feature flag status without passing in the group object to check the
feature flag against causing the attribute to always be removed unless
the feature flag was enabled globally.

The removal of the `color` attribute also happened at the wrong time.
The `color` attribute should be removed if the flag isn't enabled before
checking `args.empty?`.

Signed-off-by: Andrew Smith <espadav8@gmail.com>

ee/app/graphql/mutations/concerns/mutations/shared_epic_arguments.rb

@@ -58,13 +58,13 @@ module SharedEpicArguments
                description: 'Color of the epic. Available only when feature flag `epic_color_highlight` is enabled. This flag is disabled by default, because the feature is experimental and is subject to change without notice.'
     end
 
-    def validate_arguments!(args)
+    def validate_arguments!(args, group)
+      args.delete(:color) unless Feature.enabled?(:epic_color_highlight, group)
+
       if args.empty?
         raise Gitlab::Graphql::Errors::ArgumentError,
           'The list of epic attributes is empty'
       end
-
-      args.delete(:color) unless Feature.enabled?(:epic_color_highlight)
     end
   end
 end

ee/app/graphql/mutations/epics/create.rb

@@ -18,9 +18,10 @@ class Create < BaseMutation
       def resolve(args)
         group_path = args.delete(:group_path)
 
-        validate_arguments!(args)
-
         group = authorized_find!(group_path: group_path)
+
+        validate_arguments!(args, group)
+
         epic = ::Epics::CreateService.new(group: group, current_user: current_user, params: args).execute
 
         response_object = epic if epic.valid?

ee/app/graphql/mutations/epics/update.rb

@@ -18,9 +18,10 @@ def resolve(args)
         group_path = args.delete(:group_path)
         epic_iid = args.delete(:iid)
 
-        validate_arguments!(args)
-
         epic = authorized_find!(group_path: group_path, iid: epic_iid)
+
+        validate_arguments!(args, epic.group)
+
         epic = ::Epics::UpdateService.new(group: epic.group, current_user: current_user, params: args).execute(epic)
 
         {