fix: handle empty partitions result in integration test

sys.partitions returns empty result for unpartitioned tables.
CollectResult returns null shared_ptr when no batches are produced.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: suxiaogang223

test/inte/read_inte_test.cpp

@@ -3583,12 +3583,11 @@ TEST(SystemTableReadInteTest, TestReadGlobalPartitions) {
                                    /*ignore_if_exists=*/false));
     ArrowSchemaRelease(&schema);
 
-    ASSERT_OK_AND_ASSIGN(auto result,
+    // Unpartitioned tables are skipped by sys.partitions → empty result.
+    // CollectResult returns null shared_ptr when result is empty.
+    ASSERT_OK_AND_ASSIGN(auto part_result,
                          ReadGlobalSystemTable("partitions", catalog.get(), fs, warehouse, options));
-    auto struct_array = SingleStructChunk(result);
-    ASSERT_TRUE(struct_array);
-    // Unpartitioned tables are skipped, result should be empty
-    ASSERT_EQ(struct_array->length(), 0);
+    ASSERT_FALSE(part_result.array) << "expected null array for empty partitions result";
 }
 
 }  // namespace paimon::test