[NDTensors] Fix output type of Empty BlockSparse contraction

[mtfishman]

The output type of Empty BlockSparse contraction was a bit off, and not adjusting for the dimensions of the output tensor. That caused some subtle bugs in other operations that were expecting the types to be the same, see:

ITensor/ITensorInfiniteMPS.jl#77 (comment)

EDIT: This also adds a definition for Base.zero(T::ITensor), which makes it so that operations like Matrix{ITensor} * Matrix{ITensor} work (as matrix multiplication where the elements are ITensors that get contracted). That's helpful for MPO formats that are matrices of ITensors.

@LHerviou @kmp5VT

[mtfishman]

Closes #1127 by marking that test as broken for Julia versions 1.9 and above. @kmp5VT I know you also addressed that one in #1097 so your fix can be reverted.

[codecov-commenter]

Codecov Report

Merging #1135 (a70f612) into main (dc92810) will decrease coverage by 5.51%.
The diff coverage is 88.88%.

❗ Current head a70f612 differs from pull request most recent head 3a3023a. Consider uploading reports for the commit 3a3023a to get more accurate results

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@@            Coverage Diff             @@
##             main    #1135      +/-   ##
==========================================
- Coverage   59.61%   54.10%   -5.51%     
==========================================
  Files          87       86       -1     
  Lines        8325     8280      -45     
==========================================
- Hits         4963     4480     -483     
- Misses       3362     3800     +438     

Impacted Files	Coverage Δ
src/indexset.jl	81.47% <ø> (+21.47%)	⬆️
src/tensor_operations/tensor_algebra.jl	88.26% <66.66%> (+40.30%)	⬆️
src/broadcast.jl	86.88% <100.00%> (+59.01%)	⬆️
src/itensor.jl	80.07% <100.00%> (+31.00%)	⬆️

... and 68 files with indirect coverage changes