Handle null response output_text items

[MukundaKatta]

Summary

• allow response output_text items to carry null text values from the API
• ignore null text entries in Response.output_text
• skip structured parsing for null output_text content while still parsing valid siblings

Testing

• .venv313/bin/python -m pytest tests/lib/responses/test_responses.py -k "output_text or parse_response_skips_null_output_text" -o addopts=""
• .venv313/bin/python -c "import openai; print(openai.version)"

Closes #3063
Closes #3011