[Containers] fix support for sum(::DenseAxisArray; init)

[odow]

Related to jump-dev/MutableArithmetics.jl#339

x-ref https://discourse.julialang.org/t/jump-reports-an-error-when-attempting-to-sum-the-empty-denseaxisarray-as-the-objective-function/133332

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.86%. Comparing base (5cdd88b) to head (94ace20).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4085   +/-   ##
=======================================
  Coverage   99.86%   99.86%           
=======================================
  Files          43       43           
  Lines        5910     5910           
=======================================
  Hits         5902     5902           
  Misses          8        8           

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