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