fix(providers): handle empty choices array defensively (#6715)

[MyDude92]

Description

Resolves #6715.

Defensively handles scenarios where the OpenAI or DeepSeek API returns an empty choices list (e.g. content filtering, moderation blocks, or blank completions) instead of throwing an unhandled IndexError: list index out of range.

Changes Made

• Added defensive check: if response and response.choices and len(response.choices) > 0:
• Gracefully falls back to empty string "" matching existing provider patterns.
• Applied to both openai_provider.py and deepseek_provider.py.

Checklist

• Tested and verified
• Fixes openai and deepseek providers crash on an empty choices list #6715

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