hasPropName utility errors on null values

[chandlerprall]

If a null value gets to i18n_util.tsx::hasPropName an error is thrown

function hasPropName(child: Child): child is { propName: string } {
  return typeof child === 'object' && child.hasOwnProperty('propName');
}

This should be reproduced with a unit test before fixing. Haven't tested, but I believe this will error:

<EuiI18n token="blah" default="{arg}" values={{ arg: null }}

Originally posted by @chandlerprall in #2891