bc7_litcovid returns None instances

[jason-fries]

Describe the bug

bc7_litcovid generates instances with out a text field, e.g., {'id': '34', 'document_id': '34219343', 'text': None, 'labels': ['Prevention']}

Steps to reproduce the bug

Iterate through the dataset as normal.

Expected results

text should have a value (since it has labels)

Actual results

text is None in several cases

[shamikbose]

#self-assign

[shamikbose]

@jason-fries There's missing data in the actual file. Should I just log it as a warning and exclude it (and others like it) from the dataset?

PMID	Journal	Title	Abstract	Keywords	Publication Type	Authors	DOI	Label
34219343	Liver Int	Management of liver disease in Italy after one year of the SARS-CoV-2 pandemic: A web-based survey.		covid19;hcc;sars-cov-2;cirrhosis;liver transplant	Journal Article	Ponziani, Francesca Romana;Aghemo, Alessio;Cabibbo, Giuseppe;Masarone, Mario;Montagnese, Sara;Petta, Salvatore;Russo, Francesco Paolo;Lai, Quirino	10.1111/liv.14998	Prevention

[shamikbose]

@jason-fries This is completed in #727