[issue #454][AIC-py] get default model if no prompt model (#600)

[issue #454][AIC-py] get default model if no prompt model

Reproduced error
Confirmed now this uses gpt-3.5-turbo:

```
"default_model": "gpt-3.5-turbo"

{
   "name": "get_activities",
   "input": "Tell me 2 fun attractions to do in {{city}}.",
   "metadata": {
     "parameters": {
       "city": "New York City"
     }
   }
}
```

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* __->__ #600
* #599

python/src/aiconfig/schema.py

@@ -225,7 +225,7 @@ def get_model_name(self, prompt: Union[str, Prompt]) -> str:
         if not prompt:
             raise Exception(f"Prompt '{prompt}' not found in config.")
 
-        if not prompt.metadata:
+        if not prompt.metadata or not prompt.metadata.model:
             # If the prompt doesn't have a model, use the default model
             default_model = self.metadata.default_model
             if not default_model: