fix: fix io handling in structural_simplify, input_output_handling tests

Author: AayushSabharwal

src/systems/systemstructure.jl

@@ -630,7 +630,7 @@ function _structural_simplify!(state::TearingState, io; simplify = false,
     if has_io
         ModelingToolkit.markio!(state, orig_inputs, io...)
     end
-    if io !== nothing || any(isinput, state.fullvars)
+    if io !== nothing
         state, input_idxs = ModelingToolkit.inputs_to_parameters!(state, io)
     else
         input_idxs = 0:-1 # Empty range

test/input_output_handling.jl

@@ -50,7 +50,7 @@ end
 @test !is_bound(sys31, sys1.v[2])
 
 # simplification turns input variables into parameters
-ssys = structural_simplify(sys)
+ssys, _ = structural_simplify(sys, ([u], []))
 @test ModelingToolkit.isparameter(unbound_inputs(ssys)[])
 @test !is_bound(ssys, u)
 @test u ∈ Set(unbound_inputs(ssys))
@@ -236,7 +236,7 @@ i = findfirst(isequal(u[1]), out)
 @variables x(t) u(t) [input = true]
 eqs = [D(x) ~ u]
 @named sys = ODESystem(eqs, t)
-@test_nowarn structural_simplify(sys)
+@test_nowarn structural_simplify(sys, ([u], []))
 
 #=
 ## Disturbance input handling