fix(search): align ft.search with server

as per the ft.search docs ( https://redis.io/docs/latest/commands/ft.search ):

    If a relevant key expires while a query is running,
    an attempt to load the key's value will return a null array.
    However, the key is still counted in the total number of results.

So, instead of crashing when seeing a null as a value, we
return empty object.

fixes redis#2772
see redis#2814

Author: nkaradzhov

packages/search/lib/commands/SEARCH.ts

@@ -221,6 +221,10 @@ export interface SearchReply {
 function documentValue(tuples: any) {
   const message = Object.create(null);
 
+  if(!tuples) {
+    return message;
+  }
+
   let i = 0;
   while (i < tuples.length) {
       const key = tuples[i++],