I wanted to filter a date in my Tablix to be greater or equal to than the parameter date. The only thing was, the parameter may be null. I tried to think ahead to handle this case, and use the ISNOTHING() function to insert an early date.
Eventually I found that SSRS is able to handle this for the greater or equal to case. When the date parameter is NULL, it simply accepts all records. So inside of SSRS, NULL must be represented as a very early date.
However, this didn’t work for the upper range. When the upper date parameter range was NULL, it returned no records. So this upper date needs to be handled in the filter.
For the expression I used:
ISNOTHING( Parameters!ActionEndDate.Value ),