Likely you cannot leave the parameter blank.
You have to choose a value that is equal to blank. THis depends on the stat type of your field. Since you want a range I will guess it is numeric so use zero. If the
in your select statement you then do an OR statment
({?Param1}=0 and {?Param2}=0) OR
field in {?Param1}to {?Param2}