[BUG] `BlockSparseVector` displays sparse zero as `0` instead of `.`

As mentioned in #73 . Of course this is low priority but would be nice to fix.
```julia
bsa = BlockSparseArray{Float64}(undef,[2,2])
eachblockstoredindex(bsa)
display(eachblockstoredindex(bsa))
display(bsa)
```
```julia
0-element Dictionaries.Dictionary{CartesianIndex{1}, Block{1, Int64}}

2-blocked 4-element BlockSparseVector{Float64, Vector{Float64}, …, …}:
 0.0
 0.0
 ───
 0.0
 0.0