Try this:
right click the tablix column and select Expression.
=Fields!FirstName.Value & IIF(Fields!FirstName.Value = "", "", "; ") &
Fields!MidName.Value & IIF(Fields!MidName.Value = "", "", "; ") &
Fields!LastName.Value & IIF( Fields!LastName.Value = "", "", "; ") &
to check is null or empty:
IIF(Fields!NursingActionsClose.Value="","ifNulltakeThisPart","ifNOtNullTakeThisPart")