Fill NA

>>> from techminer2.database.field_operators import FillNAOperator
>>> (
...     FillNAOperator()  
...     #
...     # FIELDS:
...     .with_field("author_keywords")
...     .with_other_field("index_keywords")
...     #
...     # DATABASE:
...     .where_root_directory_is("example/")
...     #
...     .run()
... )