Flux.trainable - Handling trivial return case

[dynamic-queries]

Issue :
From the example for the FNO, collecting parameters throws an error as such.

Potential cause: In https://github.com/SciML/OperatorLearning.jl/blob/master/src/FourierLayer.jl#L55-L56 and subsequently https://github.com/FluxML/Flux.jl/blob/master/src/utils.jl#L500-L502, there is trivial Bool return that FourierLayer.jl does not handle, therefore throwing an error.