Fix ReactTestInstance::toJSON() with empty top-level components

[alexkirsz]

Since ReactTestInstance::toJSON() doesn't support the case where getHostComponentFromComposite returns null (The component has rendered an empty node), the following code throws:

function Empty() {
  return null;
}
ReactTestRenderer.create(<Empty />).toJSON();

This patch makes ReactTestInstance::toJSON() return null when the top-level component has rendered an empty node.

[aweary]

Seems reasonable to me! Good catch 👍 cc @spicyj

[sophiebits]

Seems all right to me, thank you!