do a nullcheck for navigator. default to null for ios_version

[gkatsev]

This allows us to require videojs in node or tonicdev.com

const browser = require('./browser.js');
console.log(browser);
{ IS_IPAD: false,
  IS_IPHONE: false,
  IS_IPOD: false,
  IS_IOS: false,
  IOS_VERSION: null,
  IS_ANDROID: false,
  ANDROID_VERSION: null,
  IS_OLD_ANDROID: false,
  IS_NATIVE_ANDROID: false,
  IS_FIREFOX: false,
  IS_EDGE: false,
  IS_CHROME: false,
  IS_IE8: false,
  IE_VERSION: null,
  TOUCH_ENABLED: false,
  BACKGROUND_SIZE_SUPPORTED: false }

/cc @ljharb

[misteroneill]

There's another navigator.userAgent here.

[misteroneill]

LGTM