feat: simplify initialization system with fully_determined=nothing …

…by default

src/systems/diffeqs/abstractodesystem.jl

@@ -1295,7 +1295,7 @@ function InitializationProblem{iip, specialize}(sys::AbstractODESystem,
         check_length = true,
         warn_initialize_determined = true,
         initialization_eqs = [],
-        fully_determined = false,
+        fully_determined = nothing,
         check_units = true,
         kwargs...) where {iip, specialize}
     if !iscomplete(sys)
@@ -1313,6 +1313,10 @@ function InitializationProblem{iip, specialize}(sys::AbstractODESystem,
                 sys; u0map, initialization_eqs, check_units, pmap = parammap); fully_determined)
     end
 
+    if !isempty(StructuralTransformations.singular_check(get_tearing_state(isys)))
+        @warn "Since the initialization system is singular, the guess values may significantly affect the initial values of the ODE"
+    end
+
     uninit = setdiff(unknowns(sys), [unknowns(isys); getfield.(observed(isys), :lhs)])
 
     # TODO: throw on uninitialized arrays

src/systems/problem_utils.jl

@@ -498,7 +498,7 @@ function process_SciMLProblem(
         constructor, sys::AbstractSystem, u0map, pmap; build_initializeprob = true,
         implicit_dae = false, t = nothing, guesses = AnyDict(),
         warn_initialize_determined = true, initialization_eqs = [],
-        eval_expression = false, eval_module = @__MODULE__, fully_determined = false,
+        eval_expression = false, eval_module = @__MODULE__, fully_determined = nothing,
         check_initialization_units = false, tofloat = true, use_union = false,
         u0_constructor = identity, du0map = nothing, check_length = true,
         symbolic_u0 = false, warn_cyclic_dependency = false,