Fix array_mergev() semantics when passed an empty array

Summary:

• If you view "active revisions" and have some revisions in one panel but no

revisions in the other panel, we'll generate a call to array_mergev(array()),
which returns null instead of array().

• This cascades into a handle lookup failure because other arrays along the

way get nulled out.

• We should probably have more strict parameter checking in array_mergev(),

but this did emit piles of warnings already, it was just a nonobvious repro (one
panel with diffs, one without).

Test Plan: Looked at a panel-with-diffs + panel-without, no exception, no
errors. Ran unit test.

Reviewers: nh, btrahan

Reviewed By: nh

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1664