fix: OpenAIChatGenerator - do not pass tools to the OpenAI client when none are provided

[anakin87]

Related Issues

In the OpenAIChatGenerator, when no tools are provided, we pass None to the client.
This is problematic for OpenAI-compatible APIs which do not support tools.
This issue was encountered in deepset-ai/haystack-core-integrations#1274

Proposed Changes:

• do not pass tools to the OpenAI client when none are provided

How did you test it?

CI, adapted existing tests to check whether we pass tools or not

Checklist

• I have read the contributors guidelines and the code of conduct
• I have updated the related issue with new insights and changes
• I added unit tests and updated the docstrings
• I've used one of the conventional commit types for my PR title: fix:, feat:, build:, chore:, ci:, docs:, style:, refactor:, perf:, test: and added ! in case the PR includes breaking changes.
• I documented my code
• I ran pre-commit hooks and fixed any issue

[coveralls]

Pull Request Test Coverage Report for Build 12709332184

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.001%) to 91.1%

---

Files with Coverage Reduction	New Missed Lines	%
components/generators/chat/openai.py	2	96.27%

Totals
Change from base Build 12708466879:	0.001%
Covered Lines:	8660
Relevant Lines:	9506

---

💛 - Coveralls