If by null you mean an empty string, you don’t need a regular expression to test this. You could use the string equal command:
string equal $iam_18 “”
will return 1 if $iam_18 is “” otherwise it’ll return 0
If you really want to use a regular expression:
regexp {^$} $iam_18
I think what your code is doing is checking if the string matches the null character (octal 0 on the ASCII chart), not checking to see if the string is empty.