Fix blockstore empty panic (bp #11423)

[mergify]

This is an automated backport of pull request #11423 done by Mergify

Cherry-pick of 1061b50 has failed:

On branch mergify/bp/v1.2/pr-11423
Your branch is up to date with 'origin/v1.2'.

You are currently cherry-picking commit 1061b5066.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:

	modified:   ledger/src/blockstore_db.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)

	both modified:   ledger/src/blockstore.rs

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[codecov]

Codecov Report

Merging #11429 into v1.2 will decrease coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff            @@
##            v1.2   #11429     +/-   ##
========================================
- Coverage   81.8%    81.8%   -0.1%     
========================================
  Files        315      315             
  Lines      75330    75339      +9     
========================================
+ Hits       61673    61679      +6     
- Misses     13657    13660      +3