[webauthn] Fix favicons not showing on autofill

If webauthn credentials were the only available credentials on the
device (i.e. there were no passwords saved), then
PasswordAutofillManager::OnAddPasswordFillData was never called. This in
turn meant that RequestFavicon was not called, so the favicon was empty.

This CL changes the behaviour to have
ContentPasswordManagerDriver::SetPasswordFillData call
PasswordAutofillManager::OnAddPasswordFillData even if the fill data is
empty, and havePasswordAutofillManager::OnAddPasswordFillData explicitly
check for that case so it has a chance to request a favicon.

Fixed: 1370037
Change-Id: I679d39b06e40b9bb11238b65397207adc398ae25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3932362
Auto-Submit: Nina Satragno <nsatragno@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Vasilii Sukhanov <vasilii@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1055211}