Custom:button-card variables: how to overload the default value?

custom:button-card is so great - especially the variables stuff.
In the templates we can configure variables with default values.
If we don’t specify any variable while using the template, the default value is taken.
Up to here common knowledge.

Does anyone know how to explicitly trigger the default value?

I’ve got a template where I have to give variables to an other button-card (a sub-card in a custom_field):

        type: custom:button-card
        template: '[[[ return variables.tmpl1 ]]]'
          enty: '[[[ return variables.card1; ]]]'
          name: '[[[ return variables.name1; ]]]'
          batt: '[[[ return variables.batt1; ]]]'
          show: '[[[ if (variables.show1 < 3) return variables.show1; ]]]'

The problem lies within the of the sub-card: it has a reasonable default, but if I enter a value in variables.show1 of the main card, I want to pass it down.

The way it is written now, something gets passed down to the sub-card, even if the javascript for show doesn’t return anything - and the default value of the sub-card is ignored since something still gets passed on.

How can I avoid this or check for this something in the sub-card’s template?