One option is to click on File > Options and set the options to convert nulls to their default values. That clears up most problems. The second option is to use IsNull() in your record selection formula. When doing this, the IsNull() function must be listed before all the other conditions. Something like this should get you on the right track.

(IsNull({client.InDateStart})  OR IsNull({client.InDateStart})) OR {?Parameter1} IN {client.InDateStart} TO {client.InDateEnd}

I cover tips and tricks for handling null values and advanced record selection formulas in my Encyclopedia book. You can find out more about my books at Amazon.com or reading the Crystal Reports eBooks online.

Although a field appears blank, it is not necessarily null.

Try creating a formula where you set the value to your date field.
Then use the formula field on your report vs. the date field.

Also, make sure you are not dealing with a DB Linking issue.
Depending on your data struture, and what fields you are linking by
it could appear to be a null field issue when all you need to do is
is change your join type to left outer.