Summary:Passing `undefined` or `null` to `clearImmediate` caused apps to crash. It it caused because we try to find the index of the null/undefined timer when we should just do nothing when passed these values.
It is already handled properly in the other Timer functions.
**Test plan**
Calling `clearImmediate` with `undefined` or `null` should do nothing.
Closes https://github.com/facebook/react-native/pull/6192
Differential Revision: D2987778
Pulled By: vjeux
fb-gh-sync-id: 6fd38cfa3c10012caa2afb27cbdab95df696a769
shipit-source-id: 6fd38cfa3c10012caa2afb27cbdab95df696a769
44fcf22074