fix: reducing over empty collection

[avik-pal]

Before the patch

julia> Functors.fleaves((; a = (;), b=[1, 2]))
ERROR: ArgumentError: reducing over an empty collection is not allowed; consider supplying `init` to the reducer
Stacktrace:
      ⋮ internal @ Base, Unknown
  [9] reduce(op::Function, itr::@NamedTuple{}; kw::@Kwargs{})
    @ Base ./reduce.jl:487
 [10] (::Functors.FlattenWalk)(::Function, ::@NamedTuple{})
    @ Functors /mnt/research/lux/Functors.jl/src/walks.jl:318
 [11] (::Functors.ExcludeWalk{Functors.FlattenWalk, Functors.var"#52#53", typeof(Functors.isleaf)})(::Function, ::@NamedTuple{})
    @ Functors /mnt/research/lux/Functors.jl/src/walks.jl:144
 [12] (::Functors.var"#recurse#26"{Functors.ExcludeWalk{Functors.FlattenWalk, Functors.var"#52#53", typeof(Functors.isleaf)}})(xs::@NamedTuple{})
    @ Functors /mnt/research/lux/Functors.jl/src/walks.jl:52
      ⋮ internal @ Unknown
 [14] map(::Function, ::@NamedTuple{a::@NamedTuple{}, b::Vector{Int64}})
    @ Base ./namedtuple.jl:266
 [15] _map
    @ /mnt/research/lux/Functors.jl/src/walks.jl:3 [inlined]
 [16] (::Functors.FlattenWalk)(::Function, ::@NamedTuple{a::@NamedTuple{}, b::Vector{Int64}})
    @ Functors /mnt/research/lux/Functors.jl/src/walks.jl:317
 [17] (::Functors.ExcludeWalk{Functors.FlattenWalk, Functors.var"#52#53", typeof(Functors.isleaf)})(::Function, ::@NamedTuple{a::@NamedTuple{}, b::Vector{Int64}})
    @ Functors /mnt/research/lux/Functors.jl/src/walks.jl:144
 [18] execute(::Functors.ExcludeWalk{Functors.FlattenWalk, Functors.var"#52#53", typeof(Functors.isleaf)}, ::@NamedTuple{a::@NamedTuple{}, b::Vector{Int64}})
    @ Functors /mnt/research/lux/Functors.jl/src/walks.jl:53
 [19] fleaves(x::@NamedTuple{a::@NamedTuple{}, b::Vector{Int64}}; exclude::Function)
    @ Functors /mnt/research/lux/Functors.jl/src/maps.jl:33
 [20] top-level scope
    @ REPL[4]:1
 [21] top-level scope
    @ none:1
Use `err` to retrieve the full stack trace.

PR Checklist

• Tests are added
• Documentation, if applicable