[core] Fix that FilesTable cannot output level0 files when dv enabled (#6414)

Author: yuzelin

paimon-core/src/main/java/org/apache/paimon/table/system/FilesTable.java

@@ -274,6 +274,9 @@ private TableScan.Plan tablePlan(FileStoreTable storeTable) {
                             return levelPredicate.test(
                                     GenericRow.of(null, null, null, null, null, level));
                         });
+            } else {
+                // avoid that batchScanSkipLevel0 is true
+                scan.withLevelFilter(level -> true);
             }
             return scan.plan();
         }

paimon-flink/paimon-flink-common/src/test/java/org/apache/paimon/flink/BatchFileStoreITCase.java

@@ -1016,4 +1016,13 @@ public void testBatchReadSourceWithoutSnapshot() {
                                 "SELECT * FROM T /*+ OPTIONS('scan.dedicated-split-generation'='true') */"))
                 .hasSize(0);
     }
+
+    @Test
+    public void testLevel0FileCanBeReadForFilesTable() {
+        sql(
+                "CREATE TABLE test_table (a int PRIMARY KEY NOT ENFORCED, b string) "
+                        + "WITH ('deletion-vectors.enabled' = 'true', 'write-only' = 'true');");
+        sql("INSERT INTO test_table VALUES (1, 'A')");
+        assertThat(sql("SELECT * FROM `test_table$files`")).isNotEmpty();
+    }
 }