Protect better against createElement(null/undefined)

This adds a warning to React.createElement in __DEV__ about using null
or undefined. This is technically valid since element creation can be
considered safe and usable in multiple rendering environments. But
rendering in a DOM environment with an element with null/undefined type
is not safe.
Merge pull request #2726 from zpao/createElement-undefined-null

Protect better against createElement(null/undefined)