feat: implement SciMLBase.check_error for null integrator

Author: AayushSabharwal

src/solve.jl

@@ -711,6 +711,7 @@ function step!(integ::NullODEIntegrator, dt = nothing, stop_at_tdt = false)
     return nothing
 end
 function SciMLBase.u_modified!(integ::NullODEIntegrator, u) end
+SciMLBase.check_error(integ::NullODEIntegrator) = integ.sol.retcode
 
 function hack_null_solution_init(prob)
     if SciMLBase.has_initialization_data(prob.f)

test/downstream/null_de.jl

@@ -93,4 +93,5 @@ end
     prob = ODEProblem(Returns(nothing), nothing, (0.0, 1.0))
     integ = init(prob, Tsit5())
     @test_nowarn SciMLBase.u_modified!(integ, Float64[])
+    @test SciMLBase.successful_retcode(SciMLBase.check_error(integ))
 end