Passing nulls and blank values to SSRS via an URL requires a very little bit of work. For a null value you first need to call up the Report Properties (in the Layout tab select the entire report and click on Properties on the right) in SSRS and click on the button in ReportParameters. Once the Report Parameters window has opened you can set the parameter to allow nulls by checking the ‘Allow null value’ box. Then from the application that runs the report use Parameter:isNull=True in your URL if you need to pass a Null value.
For a blank value call up the Report Parameters window again and check the ‘Allow blank value’ checkbox for the parameter(s) you need to be able to pass a blank value to. In your URL you can then use Parameter= to pass nothing. Here is an example URL with the Date parameter using Null, PlayerID with a blank value and Sport using a value:
&Date:isNull=True&PlayerID=&Sport=Soccer