Drop warning for empty choice content

[declark1]

There are scenarios where this is valid, so this PR drops the warning log message when there is no choice content as this is fine.

e.g.

• The first message's choice only includes role: assistant with no content
• The final message with the stop reason may not include content
• Likely other scenarios

[mdevino]

LGTM.

[declark1]

Discussed further offline, so going ahead and merging this.

In the future, if we want to send warnings to the user for streaming similar to unary in #402, it will need to be done at the end and sent with the last message, similar to whole doc output detections, after all chat completion chunks have been received and chat completion state is final. We won’t know if the entire content for each choice is empty or not until this point.