isdispatchtuple inconsistency for `typeof(Union{})`

```
julia> A = Tuple{typeof(identity), Type{Union{}}}
Tuple{typeof(identity), Type{Union{}}}

julia> B = Tuple{typeof(identity), typeof(Union{})}
Tuple{typeof(identity), Core.TypeofBottom}

julia> A == B
true

julia> Base.isdispatchtuple(A)
true

julia> Base.isdispatchtuple(B)
false
```

This was the root cause of one of the PkgEval failures in #45272.
