Improve showing unstored values

[mtfishman]

Followup to ITensor/SparseArraysBase.jl#13, also relies on ITensor/SparseArraysBase.jl#14. With this PR block sparse arrays now print like this:

julia> using BlockSparseArrays: BlockSparseArray

julia> a = BlockSparseArray{Float64}([2, 2], [2, 2])
2×2-blocked 4×4 BlockSparseArray{Float64, 2, Matrix{Float64}, SparseArraysBase.SparseArrayDOK{Matrix{Float64}, 2, BlockSparseArrays.GetUnstoredBlock{Tuple{BlockArrays.BlockedOneTo{Int64, Vector{Int64}}, BlockArrays.BlockedOneTo{Int64, Vector{Int64}}}}}, Tuple{BlockArrays.BlockedOneTo{Int64, Vector{Int64}}, BlockArrays.BlockedOneTo{Int64, Vector{Int64}}}}:
 ⋅  ⋅  │  ⋅  ⋅
 ⋅  ⋅  │  ⋅  ⋅
 ──────┼──────
 ⋅  ⋅  │  ⋅  ⋅
 ⋅  ⋅  │  ⋅  ⋅

julia> a[1, 2] = 12
12

julia> a
2×2-blocked 4×4 BlockSparseArray{Float64, 2, Matrix{Float64}, SparseArraysBase.SparseArrayDOK{Matrix{Float64}, 2, BlockSparseArrays.GetUnstoredBlock{Tuple{BlockArrays.BlockedOneTo{Int64, Vector{Int64}}, BlockArrays.BlockedOneTo{Int64, Vector{Int64}}}}}, Tuple{BlockArrays.BlockedOneTo{Int64, Vector{Int64}}, BlockArrays.BlockedOneTo{Int64, Vector{Int64}}}}:
 0.0  12.0  │  ⋅  ⋅
 0.0   0.0  │  ⋅  ⋅
 ───────────┼──────
  ⋅     ⋅   │  ⋅  ⋅
  ⋅     ⋅   │  ⋅  ⋅

while before unstored values were printed as zeros, i.e. 0.0.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 20 lines in your changes missing coverage. Please review.

Project coverage is 26.98%. Comparing base (6019b92) to head (79c770a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...locksparsearray/wrappedabstractblocksparsearray.jl	0.00%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #22      +/-   ##
==========================================
- Coverage   27.61%   26.98%   -0.63%     
==========================================
  Files          26       26              
  Lines         862      882      +20     
==========================================
  Hits          238      238              
- Misses        624      644      +20     

Flag	Coverage Δ
docs	26.90% <0.00%> (-0.63%)	⬇️

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