Fix an error with Remarkup when a header-indicator row has more cells than the one above it

Summary:
From fishing through error logs in PHI184. When row 1 has less cells than row 2, and row 2 has a |--| section (indicating that the cell above should be a table header), we try to convert an invalid cell into a table header. PHP obliges and creates this cell, but it doesn't have 'content', which produces this error:

ERROR 8: Undefined index: content at [/core/lib/libphutil/src/markup/engine/remarkup/blockrule/PhutilRemarkupBlockRule.php:160]

Test Plan: Added a failing (well, complaining) test; made it pass.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18751