[Core] Silence unnecessary deprecation warnings

[russellb]

I noticed during testing that I was getting a lot of these deprecation
warnings about local_lora_path:

DeprecationWarning: The 'lora_local_path' attribute is deprecated
     and will be removed in a future version.
     Please use 'lora_path' instead.

The check used for emitting this warning was always True, even when the
parameter was not actually specified. It will always be in
__struct_fields__. We should be checking for a non-None value,
instead.

Signed-off-by: Russell Bryant rbryant@redhat.com

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can do one of these:

• Add ready label to the PR
• Enable auto-merge.

🚀