L0 flush: avoid short-lived allocation when checking key_range empty

[problame]

We only use keys to check if it's empty so we can bail out early. No need to collect the keys for that.

Found this while doing research for #7418

[problame]

@skyzh were you ever intending to do something special with that keys vector?

[github-actions]

2922 tests run: 2805 passed, 0 failed, 117 skipped (full report)

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Code coverage* (full report)

• functions: 32.5% (6876 of 21131 functions)
• lines: 50.2% (53660 of 106975 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
fcf5518 at 2024-06-25T12:10:09.347Z :recycle:}

[skyzh]

LGTM. The original code uses hash map, and later there is a place that requires an ordered vector. Now it's B-tree map and therefore it's fine not to convert it to vec at all.

Thanks for the fix :)