fix: Do not call assignDeep for null values.

Author: CatchABus

lib/util.js

@@ -84,7 +84,7 @@ function assignDeep(object, value) {
   const copy = {};
   for (const key in object) {
     copy[key] =
-      typeof object[key] === 'object' ? assignDeep(object[key], value) : value;
+      typeof object[key] === 'object' && !isNullish(object[key]) ? assignDeep(object[key], value) : value;
   }
   return copy;
 }