pageserver: don't call get_vectored with empty keyspace

[jcsp]

Problem

This caused a variation of the stats bug fixed by #7662. That PR also fixed this case, but we still shouldn't make redundant get calls.

Summary of changes

• Only call get in the create image layers loop at the end of a range if some keys have been accumulated

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

3048 tests run: 2915 passed, 0 failed, 133 skipped (full report)

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Flaky tests (5)

Postgres 16

• test_lock_time_tracing: release
• test_vm_bit_clear_on_heap_lock: debug

Postgres 15

• test_physical_replication: release

Postgres 14

• test_gc_aggressive: debug
• test_location_conf_churn[1]: release

Code coverage* (full report)

• functions: 31.4% (6320 of 20132 functions)
• lines: 47.3% (47622 of 100707 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
19b679f at 2024-05-10T11:09:33.407Z :recycle:}