This workaround might be a little convoluted, but I've always been of the opinion that OnBase behaves better when it can search for a value that exists, instead of looking for a (null) value:
On your Unity Form:
1. Create a hidden text field.
2. Add a custom action that if the original Unity Form field is blank, it sets the value of the new hidden field to "BLANK" (or other value of your choosing).
3. Add a second custom action that if the original Unity Form field does have a value, that value is copied to the hidden field.
On your doc comp template:
1. Adjust your placeholder to look at the new hidden field
2. Adjust your condition to look for the "BLANK" value
Like I said, it's convoluted. But I think it would work.
