fix empty static sparse error

[Roger-luo]

occasionally causing QuantumBFS/YaoArrayRegister.jl#17 to fail

A = sprand_hermitian(2) # when empty
staticsize(A)

[codecov]

Codecov Report

Merging #20 into master will increase coverage by 2.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage    70.8%   72.92%   +2.11%     
==========================================
  Files          12       12              
  Lines         483      565      +82     
==========================================
+ Hits          342      412      +70     
- Misses        141      153      +12

Impacted Files	Coverage Δ
src/staticize.jl	72.72% <100%> (+2.72%)	⬆️
src/linalg.jl	69.34% <0%> (+2.03%)	⬆️
src/arraymath.jl	71.73% <0%> (+5.07%)	⬆️
src/conversions.jl	54.11% <0%> (+23.76%)	⬆️

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