This is simple linq query

datetime dob = "05152024";
var query = from row in maxLookupData.AsEnumerable() 
     where row.Field<DateTime>("bar")) <=  dob 
select row;

In above query
If one of the row.Field(“bar”) value is null, I want to replace null value with “startDate”.
How can I do that. I tried something like this no luck

datetime dob = "05152024";
var query = from row in maxLookupData.AsEnumerable() 
     where row.IsNull(Field<DateTime>("bar")? dob ) <=  dob 
select row;

How can I handle this null check ?