Converting scale array in adjoint plan application fails for subarrays

The code `convert(typeof(x), scale)` fails when `x` is a subarray, e.g. in line

https://github.com/JuliaMath/AbstractFFTs.jl/blob/1cc9ca01f9f1968face7557c372f7a90ca26f693/src/definitions.jl#L641

It would also fail if `x` is e.g. a `Zygote.OneElement`, e.g. in the context of the plan reverse rule.

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Jax's implementation for reference: https://github.com/google/jax/blob/c3aa3a4c312826cdfb0d617e30f0467c18ab4564/jax/_src/lax/fft.py#L211. Not terribly useful here because there they have no problems with generic broadcasting array construction