It seems to work to filter companies by fields when searching for certain values. Is there also a way to filter for empty values?
I tried “” or null, but they don’t work.
Update: It works through API though, similar to filtering contacts. Although documentation says there is no filtering for companies via API 
If you like to try - example:
/companies?where%5B0%5D%5Bcol%5D=field_ID&where%5B0%5D%5Bexpr%5D=isNull
it reads like this: where[0][col]=field_ID&where[0][expr]=isNull
It looks for companies, where the field “field_ID” is null.
Unfortunately it doesn’t work in the frontend, if I try “field_ID: isNull”.