Type piracy of reduce(vcat)

[MasonProtter]

DataFrames.jl is commiting type piracy on reduce(vcat):

julia> reduce(vcat, Union{}[]; init=Int[])
Int64[]

julia> using DataFrames

julia> reduce(vcat, Union{}[]; init=Int[])
ERROR: TypeError: in keyword argument init, expected AbstractDataFrame, got a value of type Vector{Int64}
Stacktrace:
 [1] top-level scope
   @ REPL[4]:1

julia> @which reduce(vcat, Union{}[]; init=Int[])
kwcall(::NamedTuple, ::typeof(reduce), ::typeof(vcat), dfs::Union{Tuple{AbstractDataFrame, Vararg{AbstractDataFrame}}, AbstractVector{<:AbstractDataFrame}})
     @ DataFrames ~/.julia/packages/DataFrames/58MUJ/src/abstractdataframe/abstractdataframe.jl:1868

This causes real errors in other packages, notably: SciML/ModelingToolkit.jl#2965

[bkamins]

Fixed in #3457