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 https://github.com/neondatabase/neon/issues/7418

[github-actions[bot]]

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

---

Code coverage* (full report)

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

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
fcf55189004bd3119eed75e2873a97da8078700c at 2024-06-25T12:10:09.347Z :recycle:}