add empty file test cases to utils_test.cc
Hi @terryheo Can you please review this PR ? Thank you! Hi @terryheo Can you please review this PR ? Thank you! Hi @rascani  Can you please review this PR ? Thank you!
Add a couple more test cases of the empty file (`empty.txt`) to `utils_test.cc`:
- reading empty file does not return nullptr
- reading empty file results in 0 lines written to vector