Refactor generate_schema method to handle nullable relationships

lib/schemable/relationship_schema_generator.rb

@@ -84,7 +84,7 @@ module Schemable
     #
     # @return [Hash] The generated schema for the relationship.
     def generate_schema(type_name, collection: false)
-      if collection
+      schema = if collection
                  {
                    type: :object,
                    properties: {
@@ -114,6 +114,13 @@ module Schemable
                    }
                  }
                end
+
+      # Modify the schema to nullable if the relationship is in nullable
+      is_relation_nullable = @model_definition.nullable_relationships.include?(type_name)
+
+      return schema unless is_relation_nullable
+
+      @schema_modifier.add_properties(schema, { nullable: true }, 'properties.data')
     end
   end
 end