fix(blob): ensure that the share sequence is not empty

[vgonkivs]

Overview

Ensure that the sequence length is not empty.

The issue was randomly found during testing blob retrieval on robusta with @walldiss

Checklist

• New and updated code has appropriate documentation
• New and updated code has new and/or updated testing
• Required CI checks are passing
• Visual proof for any user facing features like CLI or documentation updates
• Linked issues closed with keywords

[codecov-commenter]

Codecov Report

Merging #2547 (691304a) into main (a86ddca) will decrease coverage by 0.27%.
Report is 9 commits behind head on main.
The diff coverage is 50.27%.

@@            Coverage Diff             @@
##             main    #2547      +/-   ##
==========================================
- Coverage   51.39%   51.13%   -0.27%     
==========================================
  Files         158      158              
  Lines       10304    10382      +78     
==========================================
+ Hits         5296     5309      +13     
- Misses       4556     4609      +53     
- Partials      452      464      +12     

Files Changed	Coverage Δ
blob/helper.go	47.88% <0.00%> (-2.12%)	⬇️
share/eds/byzantine/bad_encoding.go	32.63% <ø> (ø)
share/eds/byzantine/share_proof.go	45.71% <0.00%> (-4.29%)	⬇️
share/p2p/shrexnd/params.go	12.50% <ø> (ø)
share/ipld/nmt_adder.go	49.48% <20.83%> (-9.90%)	⬇️
share/p2p/shrexnd/server.go	48.12% <42.42%> (-4.82%)	⬇️
share/p2p/shrexnd/client.go	52.85% <46.93%> (+7.33%)	⬆️
core/exchange.go	34.61% <50.00%> (+0.61%)	⬆️
share/eds/retriever.go	81.35% <57.14%> (-7.60%)	⬇️
core/eds.go	65.51% <62.50%> (-7.82%)	⬇️
... and 7 more

... and 4 files with indirect coverage changes