fix: fix nothing default overrides being ignored

Author: AayushSabharwal

src/systems/diffeqs/odesystem.jl

@@ -237,12 +237,11 @@ function ODESystem(deqs::AbstractVector{<:Equation}, iv, dvs, ps;
             "`default_u0` and `default_p` are deprecated. Use `defaults` instead.",
             :ODESystem, force = true)
     end
-    defaults = todict(defaults)
-    defaults = Dict{Any, Any}(value(k) => value(v)
-    for (k, v) in pairs(defaults) if value(v) !== nothing)
+    defaults = Dict{Any,Any}(todict(defaults))
     var_to_name = Dict()
     process_variables!(var_to_name, defaults, dvs′)
     process_variables!(var_to_name, defaults, ps′)
+    defaults = Dict{Any, Any}(value(k) => value(v) for (k, v) in pairs(defaults))
 
     sysdvsguesses = [ModelingToolkit.getguess(st) for st in dvs′]
     hasaguess = findall(!isnothing, sysdvsguesses)