infinite recursion when applying `f32` on empty layer

[CarloLucibello]

julia> struct M end

julia> Flux.@layer M

julia> m = M()
M()

julia> fmap(identity, m)
M()

julia> f32(m)
ERROR: StackOverflowError:
Stacktrace:
     [1] fmap(::Function, ::M; exclude::Function, walk::Functors.DefaultWalk, cache::IdDict{Any, Any}, prune::Functors.NoKeyword)
       @ Functors ~/.julia/packages/Functors/LbNAu/src/maps.jl:7
     [2] fmap(::Function, ::M)
       @ Functors ~/.julia/packages/Functors/LbNAu/src/maps.jl:3
     [3] adapt_structure(to::Flux.FluxEltypeAdaptor{Float32}, layer::M)
       @ Main ~/.julia/packages/Flux/Mhg1r/src/layers/macro.jl:102
     [4] adapt(to::Flux.FluxEltypeAdaptor{Float32}, x::M)
       @ Adapt ~/.julia/packages/Adapt/VO8tI/src/Adapt.jl:40
     [5] (::Base.Fix1{typeof(Adapt.adapt), Flux.FluxEltypeAdaptor{Float32}})(y::M)
       @ Base ./operators.jl:1127
     [6] (::Functors.ExcludeWalk{Functors.DefaultWalk, Base.Fix1{…}, typeof(Functors.isleaf)})(::Function, ::M)
       @ Functors ~/.julia/packages/Functors/LbNAu/src/walks.jl:126
     [7] (::Functors.CachedWalk{Functors.ExcludeWalk{…}, Functors.NoKeyword, Functors.WalkCache{…}})(::Function, ::M)
       @ Functors ~/.julia/packages/Functors/LbNAu/src/walks.jl:177
     [8] execute(::Functors.CachedWalk{Functors.ExcludeWalk{…}, Functors.NoKeyword, Functors.WalkCache{…}}, ::M)
       @ Functors ~/.julia/packages/Functors/LbNAu/src/walks.jl:55
     [9] fmap(::Function, ::M; exclude::Function, walk::Functors.DefaultWalk, cache::IdDict{Any, Any}, prune::Functors.NoKeyword)
       @ Functors ~/.julia/packages/Functors/LbNAu/src/maps.jl:11
--- the above 8 lines are repeated 7870 more times ---
 [62970] fmap(::Function, ::M)
       @ Functors ~/.julia/packages/Functors/LbNAu/src/maps.jl:3
 [62971] _paramtype(::Type{Float32}, m::M)
       @ Flux ~/.julia/packages/Flux/Mhg1r/src/functor.jl:127
Some type information was truncated. Use `show(err)` to see complete types.