[py][easy] Set Deleted Outputs as None (#811)

[py][easy] Set Deleted Outputs as None










Inside the python sdk, `delete_output` previously set attribute outputs
of prompts to []. While this does delete the outputs, it also retains an
empty list as the value of prompt.output

When executing config.save(), the config would have persisted empty
arrays. This behaviour is not what we want.

Instead, set to None. When config.save() is called, None values can be
ignored and removed from the resulting aiconfig.json

## Testplan
1. Run prompt
2. delete output
3. Save config.validate that config doesn't have "outputs" field for
config

| ipynb  | resulting config |
| ------------- | ------------- |
| <img width="768" alt="Screenshot 2024-01-08 at 3 26 05 PM"
src="https://github.com/lastmile-ai/aiconfig/assets/141073967/616d8fce-bf54-488b-8cc4-57b83b76744b">
| <img width="704" alt="Screenshot 2024-01-08 at 3 27 40 PM"
src="https://github.com/lastmile-ai/aiconfig/assets/141073967/10600037-04cb-400a-919d-11a60cedbc0c">
|

came up in #791

Author: Ankush-lastmile

python/src/aiconfig/schema.py

@@ -873,7 +873,7 @@ def delete_output(self, prompt_name: str):
         """
         prompt = self.get_prompt(prompt_name)
         existing_outputs = prompt.outputs
-        prompt.outputs = []
+        prompt.outputs = None
 
         return existing_outputs