pubsub/azuresb: fix non empty slice with nil message return

[Segflow]

fixes #2930

[codecov]

Codecov Report

Merging #2931 (1033324) into master (e840397) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2931      +/-   ##
==========================================
- Coverage   69.22%   69.19%   -0.03%     
==========================================
  Files         110      110              
  Lines       11447    11448       +1     
==========================================
- Hits         7924     7922       -2     
- Misses       2876     2879       +3     
  Partials      647      647              

Impacted Files	Coverage Δ
pubsub/azuresb/azuresb.go	21.60% <0.00%> (-0.09%)	⬇️
docstore/docstore.go	87.22% <0.00%> (-2.21%)	⬇️
runtimevar/filevar/filevar.go	83.87% <0.00%> (-1.08%)	⬇️
pubsub/pubsub.go	93.00% <0.00%> (+1.04%)	⬆️
internal/retry/retry.go	100.00% <0.00%> (+7.69%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9e2accc...1033324. Read the comment docs.