Have you tried to add an input parameter and pass the NullIdentifier() built-in function?
If you want to match an empty date field you can use the NullDate() function. To know more about null values you can read this in the help file.
If you still aren't able to achieve what you want please explain more explicitly the query you wanted to do.
In that case you could set the Expand Inline parameter to Yes in the Advanced Query parameters and optionally use the If built-in If function when you pass the parameter with "null" value when appropriate.