Fix debug assert in rowgroup_char_counts_kernel

[davidwendt]

Description

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.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[mythrocks]

Makes sense. LGTM!

[vyasr]

/merge