I have very much been enjoying customizing the user experience in the view by setting attrs based on the values of certain fields. But how do we check to see if the value has not been set yet? I haven't been able to figure it out

In the example below, this div is hidden if my selection field called x_sale_services_purchased, is set to any of the 3 values, but I want it to also be invisible if x_sale_services_purchased has not yet been set to anything:

<div style="background-color: #f0f0fa; border-radius: 5px; padding: 5px;" attrs="{'invisible': ['|','|','|','|',('x_sale_services_purchased','=','s_f'),('x_sale_services_purchased','=','s_web'),('x_sale_services_purchased','=','s_seo'),('x_sale_services_purchased','=','')]}"> </div>

The part in bold does not work (no error though). I have also tried 'NULL' with no success. What is the proper way to acheive this? I feel like I'm wandering into undocumented territory, although if documentation for this exists somewhere then please point me in the right direction! =) Thank you