Better allOf handling in fromProperty (#15035)

* fix allOf handling in fromProperty

* add null check, update samples

* update dart generator to handle allof with a single ref

samples/client/petstore/ruby-autoload/lib/petstore/models/all_of_with_single_ref.rb

@@ -19,6 +19,28 @@ module Petstore
 
     attr_accessor :single_ref_type
 
+    class EnumAttributeValidator
+      attr_reader :datatype
+      attr_reader :allowable_values
+
+      def initialize(datatype, allowable_values)
+        @allowable_values = allowable_values.map do |value|
+          case datatype.to_s
+          when /Integer/i
+            value.to_i
+          when /Float/i
+            value.to_f
+          else
+            value
+          end
+        end
+      end
+
+      def valid?(value)
+        !value || allowable_values.include?(value)
+      end
+    end
+
     # Attribute mapping from ruby-style variable name to JSON key.
     def self.attribute_map
       {