When rendering changesets, discard empty draft inline comments

Summary: Ref T13513. When you load a changeset, discard all empty inlines. This is likely a more desirable behavior than keeping empty editors around, even though the rest of the pipeline generally handles them fairly well now.

Test Plan:

• Started an inline, didn't type any text or save, reloaded page.
  • Before: page restores empty editor in the same place.
  • After: we just discard this likely-pointless empty inline.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21214