In addition to checking for the specific people and date range, you have to also check for null values. So, you need to edit your selection formula. It's very important to watch where your parentheses are here or else it won't work properly. It will look something like this:
(IsNull({notestable.datefield}) or
({notestable.person} = {?Person Param} and
{notestable.datefield} >= {?StartDateParam} and
{notestable.datefield} <= {?EndDateParam}))
You MUST check for null first or else this won't work!
-Dell