Fix zero segment upload

[petuhovskiy]

Found these logs on staging:

INFO Partial backup{ttid=X/Y}: failed to upload 000000010000000000000000_173_0000000000000000_0000000000000000_sk56.partial: Failed to open file "/storage/safekeeper/data/X/Y/000000010000000000000000.partial" for wal backup: No such file or directory (os error 2)
INFO Partial backup{ttid=X/Y}:upload{name=000000010000000000000000_173_0000000000000000_0000000000000000_sk56.partial}: starting upload PartialRemoteSegment { status: InProgress, name: "000000010000000000000000_173_0000000000000000_0000000000000000_sk56.partial", commit_lsn: 0/0, flush_lsn: 0/0, term: 173 }

This is because partial backup tries to upload zero segment when there is no data in timeline. This PR fixes this bug introduced in #6530.

[github-actions]

2748 tests run: 2624 passed, 0 failed, 124 skipped (full report)

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Flaky tests (1)

Postgres 16

• test_vm_bit_clear_on_heap_lock: debug

Code coverage* (full report)

• functions: 27.9% (6380 of 22850 functions)
• lines: 46.8% (44982 of 96062 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
9d4f858 at 2024-04-04T15:29:14.226Z :recycle:}