Fix ra-data-graphql incorrectly warns about deprecated override option being set

[JonasDoe]

Problem

#10052 - the check whether the override prop was used is faulty and warns even when not appropriate.

Solution

As suggested in the comments, instead of checking whether override is != null, we check whether it has keys in it. This way, its default value - {} won't pass this check, and no warning gets logged.
I used Object.keys(override).length > 0 instead of Object.keys(override).length before I favor explicitness over the somewhat wonky JavaScript coercion. I can change this, though.

How To Test

Follow the steps described in #10052 with this fixed version.

Additional Checks

• The PR targets master for a bugfix, or next for a feature
• The PR includes unit tests (if not possible, describe why) - it's quite an effort and I'm not sure it's worth it for a behavior which will disappear once the depcrecated property gets removed, and whose impact is rather neglectable (we're talking about the condition for a log entry here 🙈 ). Additionally, the make scripts don't seem to work under Windows, so I had to switch to another platform first, or try to make it run under WSL. I could try that, but I dunno when I find time for such an undertaking.
• The PR includes one or several stories (if not possible, describe why) - no were feature changed, and a log entry isn't really something to be displayed in a story anyway
• The documentation is up to date

[slax57]

Thanks for the PR and its detailed description 🙂