Make vibrationActuator nullable

[nondebug]

Closes #178

The following tasks have been completed:

• [X] Modified Web platform tests - gets automatically checked by IDL harness.

Implementation commitment:

• [x] WebKit
• [ ] Chromium (https://bugs.chromium.org/p/chromium/issues/detail?id=)
• [ ] Gecko (https://bugzilla.mozilla.org/show_bug.cgi?id=)

[marcoscaceres]

Just confirming this is what's implemented in WebKit.