fix(wal): make TruncateAll early return work when AllowEmpty is true

[wen-coding]

Summary

• The previous TruncateAll early return (first == 0 && last == 0) only matched the AllowEmpty=false never-written case. With AllowEmpty=true, an empty WAL reports first > last, so the early return never fired and TruncateAll sent a pointless truncate request to the underlying library.
• Change to first > last which correctly covers empty states when AllowEmpty is true, currently we don't call this when AllowEmpty is false.
• Remove misleading doc comment that implied TruncateAll checks AllowEmpty itself — the check is in the underlying tidwall/wal.

Test plan

• TestTruncateAllOnEmptyLog already covers calling TruncateAll on an empty log with AllowEmpty=true — now hits the early return instead of round-tripping through sendTruncate
• All existing WAL tests pass

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Mar 26, 2026, 4:52 PM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 58.58%. Comparing base (78c53dd) to head (eec4f8a).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3124   +/-   ##
=======================================
  Coverage   58.58%   58.58%           
=======================================
  Files        2099     2099           
  Lines      173525   173525           
=======================================
+ Hits       101657   101659    +2     
+ Misses      62805    62804    -1     
+ Partials     9063     9062    -1     

Flag	Coverage Δ
sei-chain-pr	70.44% <100.00%> (?)
sei-db	70.41% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sei-db/wal/wal.go	71.28% <100.00%> (+0.66%)	⬆️

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