Fix Hash value assignment in JsonSupport#getter_fn

because current version cannot send `false` value.
If given parameter is `false`, current version does not use it.

lib/google/apis/core/json_representation.rb

@@ -39,7 +39,7 @@ module Google
             ivar_name = "@#{name}".to_sym
             lambda do |_|
               if respond_to?(:[])
-                self[name] || instance_variable_get(ivar_name)
+                self.key?(name) ? self[name] : instance_variable_get(ivar_name)
               else
                 instance_variable_get(ivar_name)
               end