If multivalue parameter is left blank then display "All" else display the values

I have a multivalue parameter that asks for initials.  They do not select from a list so they can leave it blank or add as many initials as they like.  When they enter initials, this works fine:  Join(Parameters!Initials.Value, ", ").  It display the initials in the report header with a comma separator.

If they don't enter any initials I want the expression in the report header to display "ALL."  This doesn't print anything:
=IIF(Parameters!Initials.Value is Nothing, "ALL", Join(Parameters!Initials.Value, ", "))
I've also tried:
IIF(Parameters!Initials.Value="", "ALL", Join(Parameters!Initials.Value, ", "))
IIF(IsNothing(Parameters!Initials.Value), "ALL", Join(Parameters!Initials.Value, ", "))

How do I need to write this?
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Nico BontenbalCommented:
=iif(Parameters!Initials.Count=1 and Parameters!Initials.Value(0)="","all",join(parameters!Initials.Value,", "))

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