Extjs Sencha UUID custom sorting

I am trying to implement a custom sorting of the column UUID of my grid. I would like that the sorting reflects the sorting of the database by displaying the data for example in descending order, as follows:

select * from mytable order by uuid desc;

|    id    |                uuid              | 
|----------+----------------------------------+
| 10094875 |                                  | 
| 10093749 |                                  | 
| 10094905 |                                  |       
| 10094887 |                                  |                  
| 11268062 | fffffffffffffffffffff            |                 
| 11268010 | fffffffffffffffffffff            | 
| 11267357 | ffffffffffff                     | 
| 11267356 | fffff-fffff-ffff-ffff-ffff       | 
| 11267998 | eeda671280c7397c11347cb758e36b38 | 
| 10250739 | eeda671280c7397c11347cb758e36b38 |  

So in descending order, it should appear first the white spaces/empty item and after the UUID without the dashes, then the UUID with dashes.

At the moment these are my results when sorting descending:

It is close to what I want, but as you can see the empty rows are displayed at the bottom instead of the top. This is my code:

Ext.define('Traccar.model.MyModel', {
    extend: 'Ext.data.Model',
    identifier: 'negative',

    fields: [{
        ...
    }, {
    name: 'uuid',
    type: 'string',
    sortType: function (actualValue, replaceValue, arg1, arg2) {
          if (arg1 != null & arg2 != null) {
              if (actualValue === arg1 || actualValue === arg2) {
                  return replaceValue;
              } else
                  return actualValue;
          } else if (arg1 != null) {
              if (actualValue === arg1)
                  return replaceValue;
              else
                  return actualValue;
          } else
              return actualValue;
      }
}, ..
    }],   
});

Does anyone know how can I fix it?