R1:03b88d1da32d

Don't throw when a user saves account settings without changing their profile
picture.

Summary:
Going to Settings -> Account and hitting "Save" without selecting
a file in the file dialog currently throws.

Test Plan:
Went to Settings -> Account and hit "Save" without making changes.
No exception. Then uploaded a picture normally.

Differential Revision: 30
Reviewed By: tomo
Reviewers: tomo