Fix FullClusterRestartIT.testHistoryUUIDIsAdded (#38098)

This test failed once because the index wasn't fully ready 
(ie, engine opened). This commit changes the test so that 
it waits for the index to be green before checking the history 
UUID.

Closes #34452

qa/full-cluster-restart/src/test/java/org/elasticsearch/upgrades/FullClusterRestartIT.java

@@ -68,6 +68,7 @@
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.greaterThan;
+import static org.hamcrest.Matchers.hasSize;
 import static org.hamcrest.Matchers.notNullValue;
 import static org.hamcrest.Matchers.startsWith;
 
@@ -970,10 +971,14 @@ public void testHistoryUUIDIsAdded() throws Exception {
             createIndex.setJsonEntity(Strings.toString(mappingsAndSettings));
             client().performRequest(createIndex);
         } else {
+            ensureGreenLongWait(index);
+
             Request statsRequest = new Request("GET", index + "/_stats");
             statsRequest.addParameter("level", "shards");
             Response response = client().performRequest(statsRequest);
             List<Object> shardStats = ObjectPath.createFromResponse(response).evaluate("indices." + index + ".shards.0");
+            assertThat(shardStats, notNullValue());
+            assertThat("Expected stats for 2 shards", shardStats, hasSize(2));
             String globalHistoryUUID = null;
             for (Object shard : shardStats) {
                 final String nodeId = ObjectPath.evaluate(shard, "routing.node");