backport-2.1: storage: correctly handle empty forwarded proposals #31734
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Fixes #31050.
Closes #31066.
The code assumed that an empty proposal would be a command ID with empty data, but it's also possible that the entire slice is empty (command IDs+data is our encoding; Raft itself emits an entry with completely blank data).
This is the backport of #31066, which no longer needs to be merged into master. The only changes in this PR are that I addressed my comments from #31066 (review).
Release note: None