fix sparsevec

[exaexa]

No description provided.

[exaexa]

@stelmo there are 2 other sparsevec instances in community models, can you have a look if they actually do what they are supposed to do? The problem is that sparsevec([],[],...) has defaulted to a sparse vector of Anys, which don't have a default zero, thus allocation of anything breaks.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (c510103) 91.39% compared to head (1bebba9) 91.39%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #783   +/-   ##
=======================================
  Coverage   91.39%   91.39%           
=======================================
  Files          86       86           
  Lines        2209     2209           
=======================================
  Hits         2019     2019           
  Misses        190      190           

Impacted Files	Coverage Δ
src/base/types/SBMLModel.jl	92.75% <100.00%> (ø)

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