Use default observed functions when observed is empty

Author: YingboMa

src/systems/diffeqs/abstractodesystem.jl

@@ -214,7 +214,7 @@ function DiffEqBase.ODEFunction{iip}(sys::AbstractODESystem, dvs = states(sys),
     if jac
         jac_gen = generate_jacobian(sys, dvs, ps;
                                     simplify=simplify, sparse = sparse,
-                                    expression=Val{eval_expression}, expression_module=eval_module, 
+                                    expression=Val{eval_expression}, expression_module=eval_module,
                                     checkbounds=checkbounds, kwargs...)
         jac_oop,jac_iip = eval_expression ? (@RuntimeGeneratedFunction(eval_module, ex) for ex in jac_gen) : jac_gen
         _jac(u,p,t) = jac_oop(u,p,t)
@@ -227,8 +227,9 @@ function DiffEqBase.ODEFunction{iip}(sys::AbstractODESystem, dvs = states(sys),
 
     _M = (u0 === nothing || M == I) ? M : ArrayInterface.restructure(u0 .* u0',M)
 
+    obs = observed(sys)
     observedfun = if steady_state
-        let sys = sys, dict = Dict()
+        isempty(obs) ? SciMLBase.DEFAULT_OBSERVED_NO_TIME : let sys = sys, dict = Dict()
             function generated_observed(obsvar, u, p, t=Inf)
                 obs = get!(dict, value(obsvar)) do
                     build_explicit_observed_function(sys, obsvar)
@@ -237,7 +238,7 @@ function DiffEqBase.ODEFunction{iip}(sys::AbstractODESystem, dvs = states(sys),
             end
         end
     else
-        let sys = sys, dict = Dict()
+        isempty(obs) ? SciMLBase.DEFAULT_OBSERVED : let sys = sys, dict = Dict()
             function generated_observed(obsvar, u, p, t)
                 obs = get!(dict, value(obsvar)) do
                     build_explicit_observed_function(sys, obsvar; checkbounds=checkbounds)