Update cast.js

Author: vkarpov15

lib/cast.js

@@ -68,9 +68,13 @@ module.exports = function cast(schema, obj, options, context) {
         if (val[k] == null || typeof val[k] !== 'object') {
           throw new CastError('Object', val[k], path + '.' + k);
         }
-        const discrim = getSchemaDiscriminatorByValue(context.schema, val[k][schema.options.discriminatorKey]);
-        val[k] = cast(discrim ? discrim : schema, val[k], options, context);
-
+        const discriminatorValue = val[k][schema.options.discriminatorKey];
+        if (discriminatorValue == null) {
+          val[k] = cast(schema, val[k], options, context);
+        } else {
+          const discriminatorSchema = getSchemaDiscriminatorByValue(context.schema, discriminatorValue);
+          val[k] = cast(discriminatorSchema ? discriminatorSchema : schema, val[k], options, context);
+        }
       }
     } else if (path === '$where') {
       type = typeof val;