YARN-9175. Null resources check in ResourceInfo for branch-3.0

(cherry picked from commit a0291a015c1af0ea1282849bd8fb32824d7452fa)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/webapp/dao/ResourceInfo.java

@@ -62,7 +62,7 @@ public class ResourceInfo {
 
   @Override
   public String toString() {
-    return resources.toString();
+    return getResource().toString();
   }
 
   public void setMemory(int memory) {
@@ -82,6 +82,9 @@ public class ResourceInfo {
   }
 
   public Resource getResource() {
+    if (resources == null) {
+      resources = Resource.newInstance(memory, vCores);
+    }
     return Resource.newInstance(resources);
   }
 }