fix: fix simplification of empty systems

Author: AayushSabharwal

src/systems/systemstructure.jl

@@ -928,7 +928,7 @@ function mtkcompile!(state::TearingState; simplify = false,
     time_domains = merge(Dict(state.fullvars .=> ci.var_domain),
         Dict(default_toterm.(state.fullvars) .=> ci.var_domain))
     tss, clocked_inputs, continuous_id, id_to_clock = ModelingToolkit.split_system(ci)
-    if continuous_id == 0
+    if !isempty(tss) && continuous_id == 0
         # do a trait check here - handle fully discrete system
         additional_passes = get(kwargs, :additional_passes, nothing)
         if !isnothing(additional_passes) && any(discrete_compile_pass, additional_passes)

test/odesystem.jl

@@ -1628,3 +1628,9 @@ import Test: @test
 @mtkbuild sys = System(D(x) ~ t, t)
 @test sys isa System
 end
+
+@testset "Empty system can be simplified" begin
+    @named sys = System(Equation[], t)
+    ss = mtkcompile(sys)
+    @test length(equations(ss)) == length(unknowns(ss)) == 0
+end