[firestore] Correctly handle undefined values

lib/modules/firestore/utils/serialize.js

@@ -40,7 +40,7 @@ const buildNativeArray = (array: Object[]): any[] => {
 const buildTypeMap = (value: any): any => {
   const typeMap = {};
   const type = typeOf(value);
-  if (value === null) {
+  if (value === null || value === undefined) {
     typeMap.type = 'null';
     typeMap.value = null;
   } else if (value === DELETE_FIELD_VALUE) {