Fix zero-dimensional conversion

[mtfishman]

Fixes #29.

[codecov]

Codecov Report

Attention: Patch coverage is 28.57143% with 5 lines in your changes missing coverage. Please review.

Project coverage is 27.19%. Comparing base (683b35d) to head (e544c13).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/abstractblocksparsearray/map.jl	28.57%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
- Coverage   27.25%   27.19%   -0.07%     
==========================================
  Files          29       29              
  Lines         987      993       +6     
==========================================
+ Hits          269      270       +1     
- Misses        718      723       +5     

Flag	Coverage Δ
docs	26.74% <28.57%> (-0.07%)	⬇️

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