Update pss_graph_modia!'s SelectedState definition and fix alias_elimination's edge case

Author: YingboMa

src/structural_transformation/partial_state_selection.jl

@@ -94,7 +94,10 @@ function pss_graph_modia!(structure::SystemStructure, var_eq_matching, varlevel,
         end
     end
     for var in 1:ndsts(graph)
-        if varlevel[var] !== 0 && var_eq_matching[var] === unassigned
+        dv = var_to_diff[var]
+        # If `var` is not algebraic (not differentiated nor a dummy derivative),
+        # then it's a SelectedState
+        if !(dv === nothing || (varlevel[dv] !== 0 && var_eq_matching[dv] === unassigned))
             var_eq_matching[var] = SelectedState()
         end
     end

src/systems/alias_elimination.jl

@@ -6,7 +6,10 @@ const KEEP = typemin(Int)
 
 function alias_eliminate_graph!(state::TransformationState)
     mm = linear_subsys_adjmat(state)
-    size(mm, 1) == 0 && return AliasGraph(ndsts(state.structure.graph)), mm, BitSet() # No linear subsystems
+    if size(mm, 1) == 0
+        ag = AliasGraph(ndsts(state.structure.graph))
+        return ag, ag, mm, BitSet() # No linear subsystems
+    end
 
     @unpack graph, var_to_diff = state.structure