fix: handle missing in InitializationProblem

Author: AayushSabharwal

src/systems/diffeqs/abstractodesystem.jl

@@ -1546,6 +1546,7 @@ function InitializationProblem{iip, specialize}(sys::AbstractODESystem,
     parammap = parammap isa DiffEqBase.NullParameters || isempty(parammap) ?
                [get_iv(sys) => t] :
                merge(todict(parammap), Dict(get_iv(sys) => t))
+    parammap = Dict(k => v for (k, v) in parammap if v !== missing)
     if isempty(u0map)
         u0map = Dict()
     end