Merge https://github.com/google/adk-python/pull/3462
**Please ensure you have read the [contribution guide](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) before creating a pull request.**
### Link to Issue or Description of Change
**1. Link to an existing issue (if applicable):**
- Closes: #_issue_number_
- Related: #_issue_number_
**2. Or, if no issue exists, describe the change:**
**Problem:**
When using adk in streaming mode, `usage_metadata.prompt_token_count` may be `None` which will emit log
```Invalid type NoneType for attribute 'gen_ai.usage.input_tokens' value. Expected one of ['bool', 'str', 'bytes', 'int', 'float'] or a sequence of those types```
**Solution:**
Skip setting span attribute if prompt token count is None
**Unit Tests:**
- [x] All unit tests pass locally.
_Please include a summary of passed `pytest` results._
### Checklist
- [x] I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [x] I have performed a self-review of my own code.
- [x] I have commented my code, particularly in hard-to-understand areas.
- [x] I have added tests that prove my fix is effective or that my feature works.
- [x] New and existing unit tests pass locally with my changes.
- [x] I have manually tested my changes end-to-end.
- [x] Any dependent changes have been merged and published in downstream modules.
COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/3462 from wsa-2002:prompt-token-count-may-be-none-in-streaming-mode 94666862f70ed2577d5c55485e67f6da36a57bc6
PiperOrigin-RevId: 867693355
