Define `OrderStyle` for `Union{}` (#36810)
Removes ambiguity:
```
julia> Base.OrderStyle(Union{})
ERROR: MethodError: Base.OrderStyle(::Type{Union{}}) is ambiguous. Candidates:
```
This error is relevant as with current `unique!` definition that relies on `OrderStyle` one can have a problem in corner cases. E.g.:
```
julia> [i for i in ["1"] if i isa Int]
0-element Array{Union{},1}
```
Co-authored-by: Milan Bouchet-Valat <nalimilan@club.fr>
