[GopherJS] null Test

In JavaScript, we test if a variable is null by:

if (event.state === null) {
  // do something
}

In GopherJS, the equivalent is:

if event.Get("state") == nil {
      // do something
}

For undefined test (if a variable is undefined), see [3].

Tested on:

• Ubuntu Linux 16.10
• Go 1.7.4
• Chromium Version 55.0.2883.87 Built on Ubuntu , running on Ubuntu 16.10 (64-bit)

References:

[1]	GopherJS - A compiler from Go to JavaScript (GitHub, GopherJS Playground, )