Empty or Null value display in SSRS text boxes
Call a custom function?
http://msdn.microsoft.com/en-us/library/ms155798.aspx
You could always put a case statement in there to handle different types of 'blank' data.
=IIF(ISNOTHING(CStr(Fields!MyFields.Value)) or CStr(Fields!MyFields.Value) = "","-",CStr(Fields!MyFields.Value))
Use this expression you may get the answer.
Here CStr is an default function for handling String datatypes.