Support dive on shallow rendered component

[Emmanuel Ogbizi]

When testing with enzyme, diving on shallow rendered ReactList component fails with

evaluated.TypeError: Cannot read property 'parentElement' of undefined

Most likely cause, ref which sets this.el and this.items is not evaluated on a shallow render. Can solve this using mount but that results in ungainly component snapshots.

[Emmanuel Ogbizi]

Replaced my usecase with https://github.com/bvaughn/react-window, works well enough