fix(openai): Check response text is present to avoid AttributeError

[alexander-alderman-webb]

Description

According to the user in the issue gemini can return null values for chat completion text. Verify that that the messages are not null before calling model_dump().

See the corresponding schema: https://platform.openai.com/docs/api-reference/chat/object

Issues

Closes #5071

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.96%. Comparing base (2f966c6) to head (4e41f61).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5081      +/-   ##
==========================================
+ Coverage   83.94%   83.96%   +0.01%     
==========================================
  Files         179      179              
  Lines       17948    17948              
  Branches     3193     3193              
==========================================
+ Hits        15067    15070       +3     
+ Misses       1909     1906       -3     
  Partials      972      972              

Files with missing lines	Coverage Δ
sentry_sdk/integrations/openai.py	84.84% <100.00%> (ø)

... and 1 file with indirect coverage changes