Fix regression in ActiveRecord::Result#column_type

When a column type is nil in the original column_types array, the new implementation
was returning nil instead of falling back to the default type. This caused
"undefined method 'deserialize' for nil" errors when accessing attributes.

Author: george-ma

activerecord/lib/active_record/result.rb

@@ -163,7 +163,8 @@ def column_types
         @types_hash ||= begin
           types = {}
           @columns.each_with_index do |name, index|
-            types[name] = types[index] = @column_types[index]
+            type = @column_types[index] || Type.default_value
+            types[name] = types[index] = type
           end
           types.freeze
         end