fix: handle nothing values in InitializationProblem

Author: AayushSabharwal

src/systems/diffeqs/abstractodesystem.jl

@@ -1331,7 +1331,9 @@ function InitializationProblem{iip, specialize}(sys::AbstractSystem,
     end
     if u0T != Union{}
         u0T = eltype(u0T)
-        u0map = Dict(k => if symbolic_type(v) == NotSymbolic() && !is_array_of_symbolics(v)
+        u0map = Dict(k => if v === nothing
+        nothing
+        elseif symbolic_type(v) == NotSymbolic() && !is_array_of_symbolics(v)
                          v isa AbstractArray ? u0T.(v) : u0T(v)
                      else
                          v