reactComponentExpect fails silently

```reactComponentExpect``` fails silently when passed an undefined value. It calls ```ReactTestUtils.isCompositeComponent``` with the undefined value, in which a ```render``` property is accessed, throwing a prop on undefined not found error which isn't surfaced to jest. ``` TypeError: Cannot read property 'render' of undefined at Object.ReactTestUtils.isCompositeComponent (./react/test/ReactTestUtils.js:127:23) at new reactComponentExpect (./react/test/reactComponentExpect.js:46:20) at reactComponentExpect (./react/test/reactComponentExpect.js:40:12) ``` Expecting the passed-in instance to not be undefined surfaces this issue in the test runner.