fix: handle nothing sys in namespace_expr

Author: AayushSabharwal

src/systems/abstractsystem.jl

@@ -1169,6 +1169,7 @@ end
 
 function namespace_expr(
         O, sys, n = nameof(sys); ivs = independent_variables(sys))
+    sys === nothing && return O
     O = unwrap(O)
     # Exceptions for arrays of symbolic and Ref of a symbolic, the latter
     # of which shows up in broadcasts