[AMORO-2407] Fix access data file from dashboard of non-partitioned t…

…able (apache#2408)

* fix null partition

* fix listing files of non-partitioned iceberg table

ams/server/src/main/java/com/netease/arctic/server/dashboard/MixedAndIcebergTableDescriptor.java

@@ -499,7 +499,7 @@ private CloseableIterable<PartitionFileBaseInfo> collectFileInfo(
     IcebergFindFiles manifestReader =
         new IcebergFindFiles(table).ignoreDeleted().planWith(executorService);
 
-    if (partition != null && specId != null) {
+    if (table.spec().isPartitioned() && partition != null && specId != null) {
       GenericRecord partitionData = ArcticDataFiles.data(specs.get(specId), partition);
       manifestReader.inPartitions(specs.get(specId), partitionData);
     }