Fix problem with adding Differential inline comments to the last line of a file

Summary:
We use a 'null' row to indicate the element should be appended to the end of the
table (otherwise, it is prepended to the row in question), but also derive the
table from the row. This needs more cleanup in general but fix the immediate
issue at least.

Test Plan:
Added an inline comment to the last line of a file.

Reviewed By: jungejason
Reviewers: tuomaspelkonen, jungejason, aran
CC: aran, jungejason
Differential Revision: 425