fix: fix nothing pullback for parameter object in Zygote adjoint

Author: AayushSabharwal

ext/SciMLBaseZygoteExt.jl

@@ -48,6 +48,9 @@ end
             grz = pullback(getter, sym, VA.u[j], VA.prob.p, VA.t[j])[2](Δ)
             du = [k == j ? grz[2] : zero(VA.u[1]) for k in 1:length(VA.u)]
             dp = grz[3] # pullback for p
+            if dp === nothing
+                dp = parameter_values(VA)
+            end
             dprob = remake(VA.prob, p = dp)
             du, dprob
         else