Handle nothing updates better

Author: BenChung

src/systems/imperative_affect.jl

@@ -211,7 +211,9 @@ function compile_user_affect(affect::ImperativeAffect, cb, sys, dvs, ps; kwargs.
             upd_vals = user_affect(upd_component_array, obs_component_array, ctx, integ)
 
             # write the new values back to the integrator
-            _generated_writeback(integ, upd_funs, upd_vals)
+            if !isnothing(upd_vals)
+                _generated_writeback(integ, upd_funs, upd_vals)
+            end
 
             for idx in save_idxs
                 SciMLBase.save_discretes!(integ, idx)