no widening fallback for `union!!` with `Empty` as first argument

[matthias314]

julia> union!!(Int[], [1.5])
1-element Vector{Float64}:
 1.5

julia> union!!(Empty(Vector{Int}), [1.5])
ERROR: InexactError: Int64(1.5)

The problem seems to be the definition

BangBang.jl/src/NoBang/emptycontainers.jl

Line 78 in 9f8c8a5

_union(::Empty{T}, x) where {T} = unique!!(T(x))