Do not attempt to promote to Union

Sometimes `symtype` returns a union like `Union{Float64, Int64}`
```julia
julia> @variables x::Float64
1-element Vector{Num}:
 x

julia> ModelingToolkit.symtype(ifelse(x > 0, x, 0))
Union{Float64, Int64}
```


and then 
```julia
buf = get!(() -> Any[], cachevars, T)
```
fails.

This is (very) likely to occur with JSML, which appears to type things with `Float64`

Author: baggepinnen

src/systems/nonlinear/nonlinearsystem.jl

@@ -712,6 +712,9 @@ function SciMLBase.SCCNonlinearProblem{iip}(sys::NonlinearSystem, u0map,
             k = unwrap(k)
             v = unwrap(v)
             T = symtype(k)
+            while T isa Union
+                T = promote_type(T.a, T.b)
+            end
             buf = get!(() -> Any[], cachevars, T)
             push!(buf, v)
             buf = get!(() -> Any[], cacheexprs, T)