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[bot]]

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

```diff @@ 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](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/598?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/Spaces/ConstantSpace.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/598?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL1NwYWNlcy9Db25zdGFudFNwYWNlLmps) | `58.53% <77.77%> (+1.27%)` | :arrow_up: | | [src/show.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/598?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL3Nob3cuamw=) | `97.22% <100.00%> (ø)` | |

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