[Fix](fe) Fix NPE in OlapTableSink.createPaloNodesInfo due to concurrent drop backend (#58999)

Author: bobhan1

fe/fe-core/src/main/java/org/apache/doris/planner/OlapTableSink.java

@@ -866,6 +866,9 @@ public TPaloNodesInfo createPaloNodesInfo() {
         SystemInfoService systemInfoService = Env.getCurrentSystemInfo();
         for (Long id : systemInfoService.getAllBackendIds(false)) {
             Backend backend = systemInfoService.getBackend(id);
+            if (backend == null) {
+                continue;
+            }
             nodesInfo.addToNodes(new TNodeInfo(backend.getId(), 0, backend.getHost(), backend.getBrpcPort()));
         }
         return nodesInfo;