Fix kernel launch error for cudf::io::orc::gpu::rowgroup_char_counts_kernel

[davidwendt]

Description

Fixes memcheck error found during the nightly builds found in gtest OrcWriterNumericTypeTest/0.SingleColumn

# compute-sanitizer --tool memcheck gtests/ORC_TEST --gtest_filter=OrcWriterNumericTypeTest/0.SingleColumn --rmm_mode=cuda
========= COMPUTE-SANITIZER
Note: Google Test filter = OrcWriterNumericTypeTest/0.SingleColumn
[==========] Running 1 test from 1 test suite.
[----------] Global test environment set-up.
[----------] 1 test from OrcWriterNumericTypeTest/0, where TypeParam = signed char
[ RUN      ] OrcWriterNumericTypeTest/0.SingleColumn
========= Program hit cudaErrorInvalidConfiguration (error 9) due to "invalid configuration argument" on CUDA API call to cudaLaunchKernel.
=========     Saved host backtrace up to driver entry point at error
=========     Host Frame: [0x480aa6]
=========                in /usr/lib/x86_64-linux-gnu/libcuda.so.1
=========     Host Frame:cudaLaunchKernel [0x6c358]
=========                in /conda/envs/rapids/lib/libcudart.so.11.0
=========     Host Frame:__device_stub__ZN4cudf2io3orc3gpu27rowgroup_char_counts_kernelENS_6detail11base_2dspanIiNS_11device_spanEEENS5_IKNS1_22orc_column_device_viewELm18446744073709551615EEENS4_IKNS1_13rowgroup_rowsES5_EENS5_IKjLm18446744073709551615EEE(cudf::detail::base_2dspan<int, cudf::device_span>&, cudf::device_span<cudf::io::orc::orc_column_device_view const, 18446744073709551615ul>&, cudf::detail::base_2dspan<cudf::io::orc::rowgroup_rows const, cudf::device_span>&, cudf::device_span<unsigned int const, 18446744073709551615ul>&) [0x14fccb4]

Adds a check to avoid the kernel launch if the number of strings column is zero.

Checklist

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

[davidwendt]

/merge

[vuule]

Can't believe you'd just go and fix my code without telling me :P