Remove props passed with null

It would be nice, if the Entity component filters received props to exclude null values and instead uses to the default A-Frame value. IMHO this would more conform with the way React handles null values.

This would also make it simpler to add default props passed to entities. For example:

class Camera extends Component {
  static defaultProps = {
    position: null,
  };

  render() {
    const { position } = this.props;

    const camera = {
      userHeight: 1.6,
    };

    return (
      <Entity
        position={position}
        camera={camera}
        look-controls
      />
    );
  }
}

At the moment this produces the following error at https://github.com/ngokevin/aframe-react/blob/master/src/index.js#L104: Uncaught TypeError: Cannot read property 'constructor' of null

With a okay from you @ngokevin I could build a small PR for this.

[ngokevin]

Yeah, null is a valid value. If you want to filter it out, you can remove it from your props.

[lennerd]

@ngokevin Thanks for the example! Thats what I also end up using.