fix zero shaped matmul

[GiggleLiu]

Before this patch, a zero shaped matmul will crash julia.

A possible using case of zero-shaped matmul is the block diagonal matrices/tensors that used in physics simulation. During compression, one can easily generate a zero-shaped array.

[codecov[bot]]

Codecov Report

Merging #75 (c5b05f8) into master (cedfff2) will increase coverage by 1.89%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #75      +/-   ##
==========================================
+ Coverage   83.56%   85.46%   +1.89%     
==========================================
  Files          11       11              
  Lines         566      578      +12     
==========================================
+ Hits          473      494      +21     
+ Misses         93       84       -9     

Impacted Files	Coverage Δ
src/matmul.jl	89.43% <100.00%> (+4.05%)	:arrow_up:
src/block_sizes.jl	94.91% <0.00%> (-1.70%)	:arrow_down:
src/utils.jl	90.62% <0.00%> (+3.12%)	:arrow_up:

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[chriselrod]

Yeah, seems worth having the check here. Thanks for the PR!