Assuming that the stored proc will simply return nothing if the parameters are not set correctly (both NULL or both defined, with the second date parameter equal to or greater than the first), then one option is to use the Visibility property of the Tablix and hide it whenever the parameters options are not valid for the stored proc. To help the user understand why there are no results, you could also create a text box on the body of the report with its Visibility property set the opposite way and with a message to tell the user the correct way to select the date parameters.
To Hide the tablix, something like this:
= Not (IsNothing(Parameters!StartDate.Value) And IsNothing(Parameters!EndDate.Value))
OR Parameters!EndDate.Value < Parameters.StartDate.Value
To Hide the informational text box, the reverse:
= (IsNothing(Parameters!StartDate.Value) And IsNothing(Parameters!EndDate.Value))
OR Parameters!EndDate.Value >= Parameters.StartDate.Value
I am not where I can test these, so the syntax or logic may need to be tweaked, but you can get the idea anyway.