Fix ConstantSpace Multiplication for empty coefficients

[jishnub]

After this, the following works (the Fun has an empty coefficient vector):

julia> C = Multiplication(Fun(ConstantSpace(), Float64[]), ConstantSpace())
ConcreteMultiplication : ConstantSpace → ConstantSpace
 0.0

[codecov]

Codecov Report

Patch coverage: 80.00% and no project coverage change.

Comparison is base (28eb4ea) 75.24% compared to head (3116a52) 75.24%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #598   +/-   ##
=======================================
  Coverage   75.24%   75.24%           
=======================================
  Files          80       80           
  Lines        8421     8427    +6     
=======================================
+ Hits         6336     6341    +5     
- Misses       2085     2086    +1     

Files Changed	Coverage Δ
src/Spaces/ConstantSpace.jl	58.53% <77.77%> (+1.27%)	⬆️
src/show.jl	97.22% <100.00%> (ø)

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