1.18.6: broadcasting over an empty Vector{Union{}} throws UndefVarError: D once ReverseDiff is loaded #324

Versions: ReverseDiff 1.18.6 (1.18.5 is fine), Julia 1.12.5, Linux x86_64.

Reproducer (clean environment with only ReverseDiff):

using ReverseDiff
String.(Union{}[])

1.18.6:

ERROR: UndefVarError: `D` not defined in local scope
  [1] getouttype  @ ReverseDiff src/derivatives/broadcast.jl:103
  ...
  [9] copy(::Broadcasted{ReverseDiff.TrackedStyle{1}, Tuple{Base.OneTo{Int64}}, Type{String}, Tuple{Vector{Union{}}}})
      @ ReverseDiff src/derivatives/broadcast.jl:92

1.18.5 returns Union{}[].

Cause (as far as we can tell): Vector{Union{}} matches AbstractArray{<:TrackedReal} because Union{} <: TrackedReal. So the broadcast style resolves to TrackedStyle, and in copy the new line
D = mapreduce(getouttype, promote_type, args) (broadcast.jl:92) calls
getouttype(::AbstractArray{<:TrackedReal{<:Any, D}}) where {D} = D (line 102) with D unbound.

Impact: any package that broadcasts over an empty collection whose element type is Union{}
(for example an empty JSON array read by JSON3) fails once ReverseDiff is loaded anywhere in the
process, even though no tracked values are involved. We hit it through CCBlade → ImplicitAD → ReverseDiff.

Possible fixes: exclude Union{} element types from the TrackedStyle broadcast-style rule,
or guard getouttype (e.g. getouttype(::AbstractArray{Union{}}) = Union{}).