webassembly/proxy_js: Revert back to converting Py None to JS null.

This reverts part of commit fa23e4b093, to
make it so that Python `None` converts to JavaScript `null` (and JavaScript
`null` already converts to Python `None`).  That's consistent with how the
`json` module converts these values back and forth.

Signed-off-by: Damien George <damien@micropython.org>

ports/webassembly/proxy_js.js

@@ -188,7 +188,7 @@ function proxy_convert_mp_to_js_obj_jsside(value) {
     }
     if (kind === PROXY_KIND_MP_NONE) {
         // None
-        obj = undefined;
+        obj = null;
     } else if (kind === PROXY_KIND_MP_BOOL) {
         // bool
         obj = Module.getValue(value + 4, "i32") ? true : false;