Set null as the default value of deviceProperties in PointerEventInit

Previously, the default for deviceProperties in PointerEventInit was
undefined; although in chromium a value was set using the default
constructor. This CL changes this behaviour such that the default value
for deviceProperties is null. The corresponding WPT is updated as well.

For further context, please see the discussion here:
w3c/pointerevents#495 (comment)

Bug: 330760871
Change-Id: I5becf25f76e8b8c4572a783bfba8c1c4f36f8a74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5526091
Commit-Queue: Sahir Vellani <sahir.vellani@microsoft.com>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1298623}

pointerevents/deviceproperties/pointer-event-has-device-properties-uniqueid-from-pointer-event-init.tentative.html

@@ -87,6 +87,6 @@
             height: 100,
             isPrimary: true,
         });
-        CheckDeviceId(downEventEmptyProps, INVALID_UNIQUE_ID);
+        assert_equals(downEventEmptyProps.deviceProperties, null);
     }, 'No deviceProperties in PointerEventInit');
 </script>