combine reduce_empty methods for Union{} eltypes

With #49470, these can all be dispatched to the same method now,
avoiding unnecessary code duplication for this case.

base/reduce.jl

@@ -320,6 +320,8 @@ _empty_reduce_error() = throw(ArgumentError("reducing over an empty collection i
 _empty_reduce_error(@nospecialize(f), @nospecialize(T::Type)) = throw(ArgumentError("""
     reducing with $f over an empty collection of element type $T is not allowed.
     You may be able to prevent this error by supplying an `init` value to the reducer."""))
+reduce_empty(f, ::Type{Union{}}, splat...) = _empty_reduce_error(f, Union{})
+
 
 """
     Base.reduce_empty(op, T)
@@ -339,20 +341,15 @@ is generally ambiguous, and especially so when the element type is unknown).
 
 As an alternative, consider supplying an `init` value to the reducer.
 """
-reduce_empty(::typeof(+), ::Type{Union{}}) = _empty_reduce_error(+, Union{})
 reduce_empty(::typeof(+), ::Type{T}) where {T} = zero(T)
 reduce_empty(::typeof(+), ::Type{Bool}) = zero(Int)
-reduce_empty(::typeof(*), ::Type{Union{}}) = _empty_reduce_error(*, Union{})
 reduce_empty(::typeof(*), ::Type{T}) where {T} = one(T)
 reduce_empty(::typeof(*), ::Type{<:AbstractChar}) = ""
 reduce_empty(::typeof(&), ::Type{Bool}) = true
 reduce_empty(::typeof(|), ::Type{Bool}) = false
-
-reduce_empty(::typeof(add_sum), ::Type{Union{}}) = _empty_reduce_error(add_sum, Union{})
 reduce_empty(::typeof(add_sum), ::Type{T}) where {T} = reduce_empty(+, T)
 reduce_empty(::typeof(add_sum), ::Type{T}) where {T<:SmallSigned}  = zero(Int)
 reduce_empty(::typeof(add_sum), ::Type{T}) where {T<:SmallUnsigned} = zero(UInt)
-reduce_empty(::typeof(mul_prod), ::Type{Union{}}) = _empty_reduce_error(mul_prod, Union{})
 reduce_empty(::typeof(mul_prod), ::Type{T}) where {T} = reduce_empty(*, T)
 reduce_empty(::typeof(mul_prod), ::Type{T}) where {T<:SmallSigned}  = one(Int)
 reduce_empty(::typeof(mul_prod), ::Type{T}) where {T<:SmallUnsigned} = one(UInt)