Fix array format compatibility and nil handling

- Guard validator against nil ancestor_ids (array format, no serializer)
- Coerce nil to [] in ancestor_ids setter for NOT NULL columns
- Only define before_type_cast ancestry reader for string formats
  (array format has no serializer, no string conversion needed)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: kbrock

lib/ancestry/ancestry_validator.rb

@@ -3,7 +3,7 @@
 module Ancestry
   class AncestryValidator < ActiveModel::EachValidator
     def validate_each(record, attribute, value)
-      return if value.empty?
+      return if value.nil? || value.empty?
 
       if options[:integer]
         unless value.all? { |id| id.kind_of?(Integer) && id > 0 }

lib/ancestry/instance_methods_builder.rb

@@ -43,19 +43,24 @@ def ancestors?
         alias has_parent? ancestors?
 
         def ancestor_ids=(value)
+          value = value.presence || []
           write_attribute(:#{column}, value)
           #{"ancestry_sync_parent_cache(#{parent_cache_column.inspect}, value)" if parent_cache_column || parent_association}
           #{"ancestry_sync_root_cache(#{root_cache_column.inspect}, value)" if root_cache_column || root_association}
         end
 
-        # backwards compatibility: ancestry returns the raw string
-        def #{column}
-          read_attribute_before_type_cast(:#{column})
-        end
+        #{ if format_module.build_serializer(integer_pk: integer_pk)
+          <<~RUBY
+            # backwards compatibility: ancestry returns the raw string
+            def #{column}
+              read_attribute_before_type_cast(:#{column})
+            end
 
-        def #{column}=(value)
-          super(value)
-        end
+            def #{column}=(value)
+              super(value)
+            end
+          RUBY
+        end}
 
         def ancestor_ids
           read_attribute(:#{column})