If you are trying to find out if user id exists and then pass it to a Not Null component what you can do is use the selector component and load the table where the user id is located. Then go to the Outline explorer window, right click on the component and click on add---> attribute condition (conditional expression). Once you have created one, give it a meaninful name and select the right attribute (I am guessing user id field on the table). Then on predicate option choose equal.
Now using a green OK flow bind the selctor and the not null components. And this is the important part, doule click on the green arrow between the components and uncheck Enable Default Binding. On Source (Selector) you should see an option called DataSize.
DataSize (as far as I know) returns 1 if and only if there is a match between the selector and the attribute condition you added to it (which if i am not mistaken should be user id). If it returns 1 it means that there is an user id on the table, hence the data size returning a value of 1. But IT DOES NOT return the id.
I hope this helps you.