Sanitize percentile_approx() output for empty input (rapidsai#11498)

Closes rapidsai#10856 

If all of the tdigest inputs to percentile_approx are empty, it will return a column containing null rows, as expected, but they will have unsanitary offsets. This PR checks if all the inputs are empty and returns an empty column as expected.

Authors:
  - Srikar Vanavasam (https://github.com/SrikarVanavasam)

Approvers:
  - Nghia Truong (https://github.com/ttnghia)
  - MithunR (https://github.com/mythrocks)
  - https://github.com/nvdbaranec

URL: rapidsai#11498

cpp/src/quantiles/tdigest/tdigest.cu

@@ -352,13 +352,18 @@ std::unique_ptr<column> percentile_approx(tdigest_column_view const& input,
   // output is a list column with each row containing percentiles.size() percentile values
   auto offsets = cudf::make_fixed_width_column(
     data_type{type_id::INT32}, input.size() + 1, mask_state::UNALLOCATED, stream, mr);
-  auto row_size_iter = thrust::make_constant_iterator(percentiles.size());
+  auto const all_empty_rows =
+    thrust::count_if(rmm::exec_policy(stream),
+                     input.size_begin(),
+                     input.size_begin() + input.size(),
+                     [] __device__(auto const x) { return x == 0; }) == input.size();
+  auto row_size_iter = thrust::make_constant_iterator(all_empty_rows ? 0 : percentiles.size());
   thrust::exclusive_scan(rmm::exec_policy(stream),
                          row_size_iter,
                          row_size_iter + input.size() + 1,
                          offsets->mutable_view().begin<offset_type>());
 
-  if (percentiles.size() == 0) {
+  if (percentiles.size() == 0 || all_empty_rows) {
     return cudf::make_lists_column(
       input.size(),
       std::move(offsets),

cpp/tests/quantiles/percentile_approx_test.cu

@@ -405,8 +405,7 @@ TEST_F(PercentileApproxTest, EmptyInput)
                             3,
                             cudf::test::detail::make_null_mask(nulls.begin(), nulls.end()));
 
-  // TODO: change percentile_approx to produce sanitary list outputs for this case.
-  CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(*result, *expected);
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(*result, *expected);
 }
 
 TEST_F(PercentileApproxTest, EmptyPercentiles)