Fix: Handle None message content in print_prompt_completions_sample

[AmineDiro]

print_prompt_completions_sample crashes when an assistant message has content=None, which is common for tool-call-only turns, because rich's Text.append(None) raises a TypeError. This treats None as empty text.

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Note

Low Risk
Single-line defensive change in debug/logging output only; no training or model logic affected.

Overview
Fixes a crash in training-time completion logging when chat-formatted prompts/completions include messages with content: null (e.g. tool-call-only assistant turns).

In print_prompt_completions_sample’s format_entry helper, message body text is now appended as msg["content"] or "" instead of passing None into Rich’s Text.append, which raises TypeError.

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[qgallouedec]

Thanks! better to have it in its own pr instead of as part of #6947