Search for empty fields

[aulemahal]

Is your feature request related to a problem? Please describe.
I'd like to be able to search for the assets of a catalog where a given column is empty (isnull())

Describe the solution you'd like
cat.search(my_col=None) to return lines where cat.df[my_col].is_null().

Describe alternatives you've considered
Fillna the df and search for the filling value.

Additional context
I can submit a PR.

[andersy005]

👍🏽 for adding support for this