Can't clear last name in profile form

[tyxchen]

Describe the bug
In the profile form, it's not possible to clear one's last name once submitted

To Reproduce
Steps to reproduce the behavior:

1. Go to Profile
2. Clear your last name
3. Save changes
4. Watch last name field repopulate

Expected behavior
Last name field should stay clear, signifying that the change went to the backend

Additional context
This definitely has to do with clearFormVals. Perhaps we should have a sentinel value for untouched clear fields, or we could simply just put in and document a workaround for optional fields.