yacht convert crashes when genome-to-taxid file is empty, should exit more gracefully

[mahmudhera]

Current implementation, when invoked on an empty genome-to-taxid file, produces the following output:

Traceback (most recent call last):
  File "/home/grads/sjw6257/mambaforge/envs/yacht_env/bin/yacht", line 10, in <module>
    sys.exit(main())
             ^^^^^^
  File "/home/grads/sjw6257/mambaforge/envs/yacht_env/lib/python3.11/site-packages/yacht/__init__.py", line 134, in main
    args.func(args)
  File "/home/grads/sjw6257/mambaforge/envs/yacht_env/lib/python3.11/site-packages/yacht/standardize_yacht_output.py", line 134, in main
    standardize_yacht_output.run(
  File "/home/grads/sjw6257/mambaforge/envs/yacht_env/lib/python3.11/site-packages/yacht/standardize_yacht_output.py", line 481, in run
    result = self.__to_cami(sample_name)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/grads/sjw6257/mambaforge/envs/yacht_env/lib/python3.11/site-packages/yacht/standardize_yacht_output.py", line 217, in __to_cami
    result[
TypeError: 'NoneType' object is not subscriptable