Save empty username

[muhin-g-s]

Fixes #503

Problem:
In the desktop/mobile client, you can clear the username field and save, and the user's username gets removed. In the web client this doesn't work

Cause:
In the settings save handler there was a check like if (!editableUsername) return, but this isn't quite correct, since editableUsername can be not only false, but also

Fix:
A stricter check editableUsername === false

How to test:

1. open the settings modal
2. enter an empty string instead of the username
3. save

Expected behavior:

• the username field is empty
• when reopening the settings modal, the username field is absent

[netlify]

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Name	Link
🔨 Latest commit	dcd1837

[albertoblue87-netizen]

Done