If a DOI is not correct, ya2ro fails

[dgarijo]

I used https://zenodo.org/record/6518802#.YnO73lxBx9B instead of the right DOI, and I got this error:

Traceback (most recent call last):
  File "/home/dgarijo/Documents/GitHub/ya2ro/env_3.9/bin/ya2ro", line 33, in <module>
    sys.exit(load_entry_point('ya2ro', 'console_scripts', 'ya2ro')())
  File "/home/dgarijo/Documents/GitHub/ya2ro/src/ya2ro/ya2ro.py", line 66, in main
    process_yaml(args.input)
  File "/home/dgarijo/Documents/GitHub/ya2ro/src/ya2ro/ya2ro.py", line 101, in process_yaml
    data = data_wrapper.load_yaml(yaml)
  File "/home/dgarijo/Documents/GitHub/ya2ro/src/ya2ro/data_wrapper.py", line 200, in load_yaml
    init_data = init_paper(p.input_to_vocab, data)
  File "/home/dgarijo/Documents/GitHub/ya2ro/src/ya2ro/data_wrapper.py", line 305, in init_paper
    populate_datasets(paper, input_to_vocab, data)
  File "/home/dgarijo/Documents/GitHub/ya2ro/src/ya2ro/data_wrapper.py", line 411, in populate_datasets
    if doi:
UnboundLocalError: local variable 'doi' referenced before assignmen