printing multiple param - error "parameter has no value" when no value

i have this formula to print multiple parameters in the header and it works :

WhilePrintingRecords;

StringVar Array Input := {?param};

NumberVar howmany:= count (Input);

if howmany > 0 then ', ' + Input [1] +

if howmany > 2 then ', *****'

my problem is when the parameter has no value (it means all records) the report stop with the error "Parameter has no value".

in that case i want to print blank field or 'ALL RECORDS'

do you know how ?

thanks

Ricardo