Figured a late answer is better than no answer. Sorry I didn't come back and write more of a proper answer when my followup questions went unanswered.
I still think the right answer for your case is to take a look at how you're doing the show/hide, because perhaps we can integrate it with that existing stuff.
However, technically there's an easy answer, in that you can use the Sideview ValueSetter module to supply a general purpose default value for any and all cases where the real token might be blank.
If you're doing anything as advanced as using a CustomBehavior to show/hide modules on the page, you should absolutely read up on ValueSetter. Within the app use the Navigation bar to go to "Module Documentation > The ValueSetter Modules > The ValueSetter Module".
The reason I advise this, is that for a huge range of cases where you might think you need a CustomBehavior, you can actually use ValueSetter or it's cousins ResultsValueSetter and ArrayValueSetter instead and thus avoid writing the custom JS .
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