Fix split_record for all empty strings column (#16291)

Fixes `cudf::strings::split_record` handling of an all empty strings column. This caused a kernel launch with no threads eventually reporting a CUDA error. A new gtest was added to check this condition and includes tests for `rsplit_record` as well.

Closes #16284

Authors:
  - David Wendt (https://github.com/davidwendt)

Approvers:
  - Nghia Truong (https://github.com/ttnghia)
  - Yunsong Wang (https://github.com/PointKernel)

URL: #16291

cpp/src/strings/split/split.cuh

@@ -357,6 +357,12 @@ std::pair<std::unique_ptr<column>, rmm::device_uvector<string_index_pair>> split
   auto const chars_bytes =
     get_offset_value(input.offsets(), input.offset() + strings_count, stream) -
     get_offset_value(input.offsets(), input.offset(), stream);
+  if (chars_bytes == 0) {
+    auto offsets = cudf::make_column_from_scalar(
+      numeric_scalar<int32_t>(0, true, stream), strings_count + 1, stream, mr);
+    auto tokens = rmm::device_uvector<string_index_pair>(0, stream);
+    return std::pair{std::move(offsets), std::move(tokens)};
+  }
   auto const d_offsets =
     cudf::detail::offsetalator_factory::make_input_iterator(input.offsets(), input.offset());
 

cpp/tests/strings/split_tests.cpp

@@ -307,6 +307,26 @@ TEST_F(StringsSplitTest, SplitRecordWhitespaceWithMaxSplit)
   CUDF_TEST_EXPECT_COLUMNS_EQUAL(result->view(), expected);
 }
 
+TEST_F(StringsSplitTest, SplitRecordAllEmpty)
+{
+  auto input     = cudf::test::strings_column_wrapper({"", "", "", ""});
+  auto sv        = cudf::strings_column_view(input);
+  auto delimiter = cudf::string_scalar("s");
+  auto empty     = cudf::string_scalar("");
+
+  using LCW = cudf::test::lists_column_wrapper<cudf::string_view>;
+  LCW expected({LCW{}, LCW{}, LCW{}, LCW{}});
+  auto result = cudf::strings::split_record(sv, delimiter);
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(result->view(), expected);
+  result = cudf::strings::split_record(sv, empty);
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(result->view(), expected);
+
+  result = cudf::strings::rsplit_record(sv, delimiter);
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(result->view(), expected);
+  result = cudf::strings::rsplit_record(sv, empty);
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(result->view(), expected);
+}
+
 TEST_F(StringsSplitTest, MultiByteDelimiters)
 {
   // Overlapping delimiters