The load rule condition "is populated" works by:
1) Checking if the variable inside the utag.data is not undefined
2) AND Checking if the variable is not empty.
So for example an "is populated" rule would look like:
(typeof utag.data['dom.domain'] != 'undefined' && utag.data['dom.domain'] != '')
Unfortunately, to do what you want, to do "variable exists AND is not null AND is not undefined" isn't possible using just load rules. The only real way to test for null would be to use a Pre-Loader extension that checks the variable in the data object, and then converts it to be undefined, which you can then use the "is defined"/"is populated" rules.
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