On page 6 of the AN Guide it states:-

1.23 – 1.30 You might already be a British citizen without realising it. To decide whether you already have British citizenship by descent we need details about your parents.
If the father's data is not known then if can't be entered. Explanation should be given.
Personally I wouldn't just leave it blank.
It's not critical to the application.

Isn't the father's data available from other sources (other family members, birth certificates, marriage certificates etc)

Sorry can't give anything more definitive