Disabled in dropdown

Hi all, 

I have a dropdown field and have a condition for disbaled, based on that it will be disabled or not.
--> if it's not null or empty it's working fine

--> if it's null or empty, the dropdown is not disabled and I'm able to select a value for first time. Once I select the value, the dropdown gets disabled. ( since it considers that the field is not null and it's disabled. But I want the field to be editable) Can someone please help here.  

disabled: {
if(
a!isnullormepty(ri!record[my dropdown field])
false,
true
),

},