Disregard falsy elements when walking tree

[bostrom]

This PR fixes a problem with server side rendering when rendering an array and one or more of the items in the array is falsy (but other than null). This would throw an cannot read property type of null error.

An example case when this can happen is when you do a inline conditional render:

const renderSmth = false;

render() {
  return [
    renderSmth && (<Smth key="smth" />),
    <MoreContent key="morecontent" />
  ];
}

React handles this case gracefully by disregarding falsy values, not rendering them. I would like to assume that Apollo would do the same, but that's not the case, currently it only checks if an element is null.

This PR replaces the element == null with !element, effectively disregarding all falsy values altogether, more closely mimicking the behaviour of React.

[apollo-cla]

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[rosskevin]

Thanks @bostrom

[TzviPM]

@rosskevin any idea when the next react-apollo release will be, so we can take advantage of this fix?

[rosskevin]

@omniroot We don't have a timeline yet, but if you are up for using master, feel free to grab my package for the time being (you'll have to either alias your imports or search/replace for the time being). https://www.npmjs.com/package/@alienfast/react-apollo

We will be discussing strategy for the next release on Friday 1/5/18 with the committers.

[bostrom]

@omniroot as a workaround, you can filter the array:

render() {
  return [
    renderSmth && (<Smth key="smth" />),
    <MoreContent key="morecontent" />
  ].filter(Boolean);
}

[TzviPM]

@rosskevin any idea what the plan is for the next release?

[rosskevin]

@omniroot we are discussing on a call tomorrow, call was moved from Friday.

[TzviPM]

@rosskevin any updates?