Fix an issue with 1up diff block rendering for added or removed blocks

Summary: Ref T13425. When a change adds or removes a block (vs adding or removing a document, or changing a block), we currently try to render null as cell content in the unified view. Make this check broader to catch both "no opposite document" and "no opposite cell".

Test Plan:

Subscribers: artms

Maniphest Tasks: T13425

Differential Revision: https://secure.phabricator.com/D20897