Merge pull request #21807 from veblush/submodule-at-head

No error when there is nothing to commit for submodule-at-head test

tools/internal_ci/linux/grpc_build_submodule_at_head.sh

@@ -48,6 +48,6 @@ then
 fi
 
 # commit so that changes are passed to Docker
-git -c user.name='foo' -c user.email='foo@google.com' commit -a -m 'Update submodule'
+git -c user.name='foo' -c user.email='foo@google.com' commit -a -m 'Update submodule' --allow-empty
 
 tools/run_tests/run_tests_matrix.py -f linux --inner_jobs 4 -j 4 --internal_ci --build_only