Better printing to show unstored values

[mtfishman]

Before this PR, unstored values were just printed as normal zero values, while with this PR we have:

julia> using SparseArraysBase: SparseArrayDOK

julia> a = SparseArrayDOK{Float64}(2, 2)
2×2 SparseArrayDOK{Float64, 2, typeof(SparseArraysBase.default_getunstoredindex)}:
 ⋅  ⋅
 ⋅  ⋅

julia> a[1, 2] = 3
3

julia> a
2×2 SparseArrayDOK{Float64, 2, typeof(SparseArraysBase.default_getunstoredindex)}:
 ⋅  3.0
 ⋅   ⋅

similar to the printing in SparseArrays.jl.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 65.77%. Comparing base (b4f5a37) to head (1b6d0fe).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/sparsearraydok.jl	0.00%	3 Missing ⚠️
src/abstractsparsearray.jl	77.77%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
+ Coverage   64.28%   65.77%   +1.49%     
==========================================
  Files           6        6              
  Lines         238      263      +25     
==========================================
+ Hits          153      173      +20     
- Misses         85       90       +5     

Flag	Coverage Δ
docs	65.64% <80.00%> (+1.51%)	⬆️

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