Optional multi-valued parameters in SSRS

Have a look at this article on Passing Multivalued parameters.

Basically the above link contains a workaround to meet your requirement for an optional multivalued parameter. The steps to achieve this are as follows:

  1. Replace "ALL" with " " (BLANK) in the parameter's dataset query.
  2. Assign -1 as default parameter value for your multivalued parameter.