[WebAuthn] Remove empty entry from password autofill dialog

Currently when there are Web Authentication credentials available to
be autofilled into a username field and no username/password
credentials, there is an extra entry with the text "No username"
showing in the popup. This is because the username and password fields
of PasswordFormFillData are still read, and they are empty.

This change causes the the fill data to be ignored when when those
fields are empty strings.

Fixed: 1313095
Change-Id: Ic21c0c8d5395a168c06dc0a3b8079adca7cd5d36
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3848742


Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Ken Buchanan <kenrb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1039264}