Backport: Don't treat ConstantOperator as a zeroop

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: +18.75% :tada:

Comparison is base (b5cd67e) 52.51% compared to head (d8316a8) 71.27%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## backport_0.8 #593 +/- ## ================================================= + Coverage 52.51% 71.27% +18.75% ================================================= Files 81 81 Lines 8517 8592 +75 ================================================= + Hits 4473 6124 +1651 + Misses 4044 2468 -1576 ``` | [Files Changed](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/593?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/Operators/banded/ConstantOperator.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/593?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL09wZXJhdG9ycy9iYW5kZWQvQ29uc3RhbnRPcGVyYXRvci5qbA==) | `80.68% <ø> (+20.45%)` | :arrow_up: | ... and [66 files with indirect coverage changes](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/593/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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