Fix testCancelFailedSearchWhenPartialResultDisallowed (#64248)

The search request in the test can complete before the "cancelledLatch" 
is latched if the second shard request is sent after the request was
canceled (i.e., the child task is not allowed to start after the parent
was canceled).

Closes #63976
Relates ##63520

Author: dnhatn

server/src/internalClusterTest/java/org/elasticsearch/search/SearchCancellationIT.java

@@ -281,7 +281,6 @@ public void testCancelMultiSearch() throws Exception {
         }
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/63976")
     public void testCancelFailedSearchWhenPartialResultDisallowed() throws Exception {
         final List<ScriptedBlockPlugin> plugins = initBlockFactory();
         int numberOfShards = between(2, 5);
@@ -325,8 +324,12 @@ public void testCancelFailedSearchWhenPartialResultDisallowed() throws Exception
             queryLatch.countDown();
             assertBusy(() -> {
                 final List<SearchTask> searchTasks = getSearchTasks();
-                assertThat(searchTasks, hasSize(1));
-                assertTrue(searchTasks.get(0).isCancelled());
+                // The search request can complete before the "cancelledLatch" is latched if the second shard request is sent
+                // after the request was cancelled (i.e., the child task is not allowed to start after the parent was cancelled).
+                if (searchTasks.isEmpty() == false) {
+                    assertThat(searchTasks, hasSize(1));
+                    assertTrue(searchTasks.get(0).isCancelled());
+                }
             }, 30, TimeUnit.SECONDS);
         } finally {
             for (ScriptedBlockPlugin plugin : plugins) {