🧹 Remove ENV['HYKU_IIIF_PRINT'] conditional

The knowledge of why we do or don't have `HYKU_IIIF_PRINT` is a bit
odd. I'm not excited to have an `ENV` from Hyku show up here; especially
an `ENV` that only partially removes the full functionality of IIIF
Print.

What I'm wondering is if we should have Hyku decorate the
work_resource.set_child_flag to account for it's implementation foibles
relative to how Hyku is using the HYKU_IIIF_PRINT.

Put another, when we see this variable, my read is HYKU_IIIF_PRINT ==
false means don't do anything IIIF Print related. But that is not what
it means in Hyku, thus we should limit the knowledge/foibles of that ENV
to Hyku only.

app/transactions/hyrax/transactions/iiif_print_container_decorator.rb

@@ -28,29 +28,31 @@ class IiifPrintContainerDecorator
         end
       end
 
-      if ENV['HYKU_IIIF_PRINT']
-        namespace 'change_set' do |ops|
-          ops.register 'update_work' do
-            steps = Hyrax::Transactions::WorkUpdate::DEFAULT_STEPS.dup
-            steps.insert(steps.index('work_resource.update_work_members') + 1, 'work_resource.set_child_flag')
-            Hyrax::Transactions::WorkUpdate.new(steps: steps)
-          end
+      namespace 'change_set' do |ops|
+        ops.register 'update_work' do
+          steps = Hyrax::Transactions::WorkUpdate::DEFAULT_STEPS.dup
+          # NOTE: applications that don't want the file splitting of IIIF Print will want to remove
+          # this step from their transactions.
+          steps.insert(steps.index('work_resource.update_work_members') + 1, 'work_resource.set_child_flag')
+          Hyrax::Transactions::WorkUpdate.new(steps: steps)
         end
+      end
 
-        if defined?(Bulkrax::Transactions::Container)
-          namespace 'work_resource' do |ops|
-            ops.register Bulkrax::Transactions::Container::UPDATE_WITH_BULK_BEHAVIOR do
-              steps = Bulkrax::Transactions::Container::UPDATE_WITH_BULK_BEHAVIOR_STEPS.dup
-              steps.insert(steps.index('work_resource.update_work_members') + 1, 'work_resource.set_child_flag')
-              Hyrax::Transactions::WorkUpdate.new(steps: steps)
-            end
+      if defined?(Bulkrax::Transactions::Container)
+        namespace 'work_resource' do |ops|
+          ops.register Bulkrax::Transactions::Container::UPDATE_WITH_BULK_BEHAVIOR do
+            steps = Bulkrax::Transactions::Container::UPDATE_WITH_BULK_BEHAVIOR_STEPS.dup
+            # NOTE: applications that don't want the file splitting of IIIF Print will want to remove
+            # this step from their transactions.
+            steps.insert(steps.index('work_resource.update_work_members') + 1, 'work_resource.set_child_flag')
+            Hyrax::Transactions::WorkUpdate.new(steps: steps)
           end
         end
+      end
 
-        namespace 'work_resource' do |ops|
-          ops.register 'set_child_flag' do
-            Steps::SetChildFlag.new
-          end
+      namespace 'work_resource' do |ops|
+        ops.register 'set_child_flag' do
+          Steps::SetChildFlag.new
         end
       end
     end

app/transactions/hyrax/transactions/steps/set_child_flag.rb

@@ -6,12 +6,18 @@ module Steps
       class SetChildFlag
         include Dry::Monads[:result]
 
+        # @see IiifPrint.model_configuration
         def call(resource)
           return Failure(:resource_not_persisted) unless resource.persisted?
 
           user = ::User.find_by_user_key(resource.depositor)
 
           Hyrax.custom_queries.find_child_works(resource: resource).each do |child_work|
+            # not all child works might have the is_child property that we define when we configure
+            # the a model for iiif_print.  see IiifPrint.model_configuration
+            #
+            # Put another the existence of the is_child property is optional.
+            next unless child_work.respond_to?(:is_child)
             next if child_work.is_child
             child_work.is_child = true
             Hyrax.persister.save(resource: child_work)