Set default content to "" (empty string), not null, in PhrictionDocumentEditor

Summary: phriction.edit allows you to omit the content string, meaning "don't
edit content". If you're also creating the page, we currently break in a
terrible, horrible, no-good, very-bad way because 'content' can't be null.
Default to empty string instead so phriction.edit creates an empty page instead
of a broken mess.

Test Plan: Called phriction.edit on a new page with no content.

Reviewers: skrul, jungejason, nh, tuomaspelkonen, aran

Reviewed By: skrul

CC: aran, skrul

Differential Revision: 920

src/applications/phriction/editor/document/PhrictionDocumentEditor.php

@@ -53,6 +53,7 @@ final class PhrictionDocumentEditor {
       $content = new PhrictionContent();
       $content->setSlug($slug);
       $content->setTitle($default_title);
+      $content->setContent('');
     }
 
     $obj = new PhrictionDocumentEditor();