Backport: Fix ConstantSpace Multiplication for empty coefficients

[codecov[bot]]

Codecov Report

Patch coverage: 80.00% and project coverage change: +0.01% :tada:

Comparison is base (c398dc9) 71.27% compared to head (ca9deb6) 71.29%. Report is 1 commits behind head on backport_0.8.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## backport_0.8 #599 +/- ## ================================================ + Coverage 71.27% 71.29% +0.01% ================================================ Files 81 81 Lines 8592 8600 +8 ================================================ + Hits 6124 6131 +7 - Misses 2468 2469 +1 ``` | [Files Changed](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/599?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/599?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL1NwYWNlcy9Db25zdGFudFNwYWNlLmps) | `52.89% <77.77%> (+1.56%)` | :arrow_up: | | [src/show.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/599?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL3Nob3cuamw=) | `95.37% <100.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/599/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation)

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