typeinfer should return NoneType for None (#275)

I think we should just special case `None` for now before actually
supporting enums and singletons (#266)

Author: Roger-luo

src/kirin/dialects/func/typeinfer.py

@@ -25,7 +25,10 @@ def const_none(self, interp: TypeInference, frame: Frame, stmt: ConstantNone):
 
     @impl(Return)
     def return_(self, interp: TypeInference, frame: Frame, stmt: Return) -> ReturnValue:
-        if isinstance(hint := stmt.value.hints.get("const"), const.Value):
+        if (
+            isinstance(hint := stmt.value.hints.get("const"), const.Value)
+            and hint.data is not None
+        ):
             return ReturnValue(types.Literal(hint.data))
         return ReturnValue(frame.get(stmt.value))