Create a service with default model "SDC AID" throws an Invalid model exception.

To reproduce the error:

1. Go to the service creation
2. Select other model than "SDC AID", for example "ETSI SOL001 v2.5.1"
3. Change back to "SDC AID" (here is were the value is set to empty string and the cause of the bug)
4. Fill up the rest of the mandatory fields (name, category and description)
5. Click to create a service 

The service creation will return an error: 

{
  "requestError": {
    "serviceException": {
      "messageId": "SVC4145", 
      "text": "Invalid model \u0027%1\u0027.",
      "variables": [ "" ] 
    }
  }
}