Prompt Drivers Crash When Streaming Without A Structure

[collindutter]

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Describe the bug Prompt Drivers require structure to be set in order to publish CompletionChunkEvents.

To Reproduce Steps to reproduce the behavior:

1. Create a Prompt Driver without a Structure.
2. Try streaming.
3. Observe a crash here.

Expected behavior Prompt Drivers should not depend on a Structure just to stream. Perhaps they should take in an event_listener: EventListener instead of a structure: Structure. If a Structure is the one using the Prompt Driver, it can provide an EventListener. Otherwise we can default one.

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Desktop (please complete the following information): N/A

Additional context Add any other context about the problem here.