You can implement this by changing triggers to non-mandatory ones (right-click on each trigger and uncheck Mandatory), and using Collections/Exists to test whether a value was passed, as in the following screenshot (based on my previous example),
Of-course you can do the same for more than one field, by repeating the steps and reusing the NIL if Not Exists model.
Such modelling is explained in our step-by-step tutorial. Note that while the tutorial demonstrates modelling of a desktop application, the principles of modelling are just the same as for mobile applications.
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