First, you are not sending the variable to null, you are setting it the empty string. This is a difference.
If you just want to test if it is empty, I prefer to use lempty even though it is a string. For example:
if {[lempty $iam_18]} {d something}
If something else is setting the field to a true null (Hex 00) then test it like:
if {{regexp — {x00} $iam_18]} {do something}
You really need to use empty string instead of null. Null has a different connatation.
