Trigger when numeric entry is blank

I have a slide with numeric entry fields and want to show a message layer if the user clicks Submit without filling in all the numeric entry fields.

Since there is no option for "blank" in the conditions, I set the conditions to when the entry is zero.  This only works if the user types in 0. If the fields are all blank, nothing happens. I have not found any way to trigger the layer based on the absence of an input.

I am using Storyline 360.

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