Allow streaming actions to yield just state for all but the last one

[elijahbenizzy]

Is your feature request related to a problem? Please describe.

This should work:

@streaming_action(reads=["prompt"], writes=['response'])
        def streaming_response(state: State) -> Generator[dict, None, tuple[dict, State]]:
            response = client.chat.completions.create(
                model='gpt-3.5-turbo',
                messages=[{
                    'role': 'user',
                    'content': state["prompt"]
                    }],
                temperature=0,
            )
            buffer = []
            for chunk in response:
                delta = chunk.choices[0].delta.content
                buffer.append(delta)
                # yield partial results
                yield {'response': delta}
            full_response = ''.join(buffer)
            # return the final result
            return {'response': full_response}, state.update(response=full_response)

Instead the yield needs to be {"response": delta}, None

Describe the solution you'd like
If it's not the last yield, we should just yield the result, no need to specify None for the state update.

Tasks:

1. Make the change here and in the function below:

   burr/burr/core/application.py

   Line 253 in f883885

   # TODO -- consider adding support for just returning a result.

2. Add tests for this, copy from those that use this class, and corresponding async.

Describe alternatives you've considered
Not doing it