<All other values> The symbology class will disappear from the map if we turn off the other symbology classes for Unique Values in ArcGIS Pro.

Solución alternativa

1. Click "Add all values" to include the <Null> values in the symbol classes. 
2. If user is using a domain, manually add the <Null> via Add Unlisted Values button and type in the <Null> value. 
3. The <Null> should be visible on the map even after toggling off the visibility of the other classes.

Pasos para reproducir