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>

tests/ports/webassembly/py_proxy_to_js.mjs.exp

@@ -1,4 +1,4 @@
 false 1
 true [ 1, 2, 3 ]
-true [ undefined, true, 1.2 ]
-true { tuple: [ 1, 2, 3 ], one: 1, list: [ undefined, true, 1.2 ] }
+true [ null, true, 1.2 ]
+true { tuple: [ 1, 2, 3 ], one: 1, list: [ null, true, 1.2 ] }

tests/ports/webassembly/run_python_async.mjs.exp

@@ -23,7 +23,7 @@ py 1
 setTimeout resolved
 resolved value: 123
 py 2
-2 undefined
+2 null
 = TEST 4 ==========
 1
 py 1
@@ -35,4 +35,4 @@ py 3
 setTimeout B resolved
 resolved value: 456
 py 4
-2 undefined
+2 null