[bug] Not possible to clear out the extended bio

[brandonkelly]

If you have already entered an extended for your profile, it’s not possible to clear it out.

Steps to reproduce

1. Go to your profile at https://opencollective.com/your-name
2. Enter an extended bio, if you don’t have one already (the one that says it supports Markdown). Reload the page to verify that it saved.
3. Delete the contents of the extended bio field.

Expected behavior

The bio would go back to its initial empty state.

Actual behavior

The bio doesn’t change. Further, if you try to outsmart the page by just entering a space in the bio, the placeholder text will actually get saved to it:

(Screenshot from viewing the profile in an incognito window.)