Fix debug assert in rowgroup_char_counts_kernel (#15902)

Fixes assert triggered by `OrcWriterTest.EmptyChildStringColumn` in a Debug build.

```
$ gtests/ORC_TEST --gtest_filter=OrcWriterTest.EmptyChildStringColumn
Note: Google Test filter = OrcWriterTest.EmptyChildStringColumn
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from OrcWriterTest
[ RUN      ] OrcWriterTest.EmptyChildStringColumn
/cudf/cpp/include/cudf/detail/offsets_iterator.cuh:79: cudf::detail::input_offsetalator::input_offsetalator(const void *, cudf::data_type, int): block: [0,0,0], thread: [0,0,0] Assertion `(dtype.id() == type_id::INT32 || dtype.id() == type_id::INT64) && "Unexpected offsets type"` failed.
CUDA Error detected. cudaErrorAssert device-side assert triggered
ORC_TEST: /conda/envs/rapids/include/rmm/mr/device/detail/stream_ordered_memory_resource.hpp:248: void rmm::mr::detail::stream_ordered_memory_resource<PoolResource, FreeListType>::do_deallocate(void*, std::size_t, rmm::cuda_stream_view) [with PoolResource = rmm::mr::pool_memory_resource<rmm::mr::cuda_memory_resource>; FreeListType = rmm::mr::detail::coalescing_free_list; std::size_t = long unsigned int]: Assertion `status__ == cudaSuccess' failed.
Aborted (core dumped)
```

Error introduced in #15891 where offsetalator wraps an offsets column in the `cudf::io::orc::gpu::rowgroup_char_counts_kernel`.
But when `num_rows==0` the offsets column is `EMPTY` causing the assert to trigger.
Checking the `num_rows` before accessing the offsets column fixes the issue.

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

Approvers:
  - Bradley Dice (https://github.com/bdice)
  - MithunR (https://github.com/mythrocks)

URL: #15902

cpp/src/io/orc/dict_enc.cu

@@ -44,12 +44,13 @@ CUDF_KERNEL void rowgroup_char_counts_kernel(device_2dspan<size_type> char_count
   auto const start_row = rowgroup_bounds[row_group_idx][col_idx].begin + str_col.offset();
   auto const num_rows  = rowgroup_bounds[row_group_idx][col_idx].size();
 
-  auto const& offsets    = str_col.child(strings_column_view::offsets_column_index);
-  auto const offsets_itr = cudf::detail::input_offsetalator(offsets.head(), offsets.type());
-  char_counts[str_col_idx][row_group_idx] =
-    (num_rows == 0)
-      ? 0
-      : static_cast<size_type>(offsets_itr[start_row + num_rows] - offsets_itr[start_row]);
+  size_type char_count = 0;
+  if (num_rows > 0) {
+    auto const& offsets    = str_col.child(strings_column_view::offsets_column_index);
+    auto const offsets_itr = cudf::detail::input_offsetalator(offsets.head(), offsets.type());
+    char_count = static_cast<size_type>(offsets_itr[start_row + num_rows] - offsets_itr[start_row]);
+  }
+  char_counts[str_col_idx][row_group_idx] = char_count;
 }
 
 void rowgroup_char_counts(device_2dspan<size_type> counts,