Minor refactor: Be consistent about alias_eliminate_graph! return type

Generally, it's bad style to change the return type in a corner cases,
because a lot of the time (as I happed to), users will mostly test
the common case and then when the corner case (in this case no linear
equations in the system) comes up, things break. Simplify things
by being consistent about the return types.

Author: Keno

src/systems/alias_elimination.jl

@@ -4,7 +4,7 @@ const KEEP = typemin(Int)
 
 function alias_eliminate_graph!(state::TransformationState)
     mm = linear_subsys_adjmat(state)
-    size(mm, 1) == 0 && return nothing, mm, BitSet() # No linear subsystems
+    size(mm, 1) == 0 && return AliasGraph(ndsts(state.structure.graph)), mm, BitSet() # No linear subsystems
 
     @unpack graph, var_to_diff = state.structure
 
@@ -37,7 +37,7 @@ function alias_elimination!(state::TearingState)
     sys = state.sys
     complete!(state.structure)
     ag, mm, updated_diff_vars = alias_eliminate_graph!(state)
-    ag === nothing && return sys
+    isempty(ag) && return sys
 
     fullvars = state.fullvars
     @unpack var_to_diff, graph = state.structure