Fixed CLI streamed chat completions.

[MikeAmy]

Streamed chat completions use a different response structure vs non-streamed, i.e. deltas, which caused KeyError exceptions. Also these token deltas may have roles, and there may be empty deltas at the end of a completion. Handle these cases sensibly without breaking the non-streamed path.

I manually tested both the streamed and non-streamed paths, but no unit tests, sorry, was busy.