Hide interface field if it has a null value

If a variable does not have a value save into it how do you hide it in the interface? 

This was my attempt at it using the showWhen variable: 

a!dateField(
label: "Tier 2 Findings",
labelPosition: "ABOVE",
value: ri!svcRequest.tier_two_date,
saveInto: {},
showWhen: if (ri!svcRequest.tier_two_date, true, false) ,
readOnly: true,
validations: {}
),

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