fix: fix a bug where AppendRequest with no entries triggers flush

[grobinson-grafana]

What this PR does / why we need it:

This commit fixes a bug where an AppendRequest with no entries triggers a flush. This is because the code expects a successful Append to SegmentWriter to set the firstAppend timestamp. This is an alternative fix to #13665.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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[benclive]

nice, this is much cleaner I think