feat: implement u_modified! for null integrator

Author: AayushSabharwal

src/solve.jl

@@ -710,6 +710,7 @@ function step!(integ::NullODEIntegrator, dt = nothing, stop_at_tdt = false)
     end
     return nothing
 end
+function SciMLBase.u_modified!(integ::NullODEIntegrator, u) end
 
 function hack_null_solution_init(prob)
     if SciMLBase.has_initialization_data(prob.f)

test/downstream/null_de.jl

@@ -88,3 +88,9 @@ end
     sol = solve(prob)
     @test sol.resid isa SVector{1, Float64}
 end
+
+@testset "`u_modified!` works on null integrators" begin
+    prob = ODEProblem(Returns(nothing), nothing, (0.0, 1.0))
+    integ = init(prob, Tsit5())
+    @test_nowarn SciMLBase.u_modified!(integ, Float64[])
+end