Fix issue with pattern matching empty list which interferes with boolean optimisations.

The pattern matching case  for empty list `switch x { | list{} =>` used to compile to `if(x)`, which interferes with boolean optimisation.
Now it compiles to `if(x ==0)` which is the same output as for `if(x == list{})`.

Fixes #7235

Author: cristianoc

compiler/ml/matching.ml

@@ -2245,7 +2245,8 @@ let combine_constructor sw_names loc arg ex_pat cstr partial ctx def
           let arg =
             if Datarepr.constructor_has_optional_shape cstr then
               Lprim (Pis_not_none, [arg], loc)
-            else arg
+            else
+              Lprim (Pjscomp Cneq, [arg; Lconst (Const_base (Const_int 0))], loc)
           in
           Lifthenelse (arg, act2, act1)
         | 2, 0, [(i1, act1); (_, act2)], []