Fix bug in broadcasting SparseAxisArray

[odow]

Closes #2919

[codecov]

Codecov Report

Merging #2929 (bc4ae5f) into master (7691ee2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2929   +/-   ##
=======================================
  Coverage   95.38%   95.39%           
=======================================
  Files          43       43           
  Lines        5791     5795    +4     
=======================================
+ Hits         5524     5528    +4     
  Misses        267      267           

Impacted Files	Coverage Δ
src/Containers/SparseAxisArray.jl	98.07% <100.00%> (+0.07%)	⬆️

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[odow]

Nightly failure is #2930